Amends c884bfd5 (X neo/protogen: Catch length checks overflows on decode)

See https://github.com/golang/go/issues/19126#issuecomment-358743715.

Examples:

		before										after

TEXT ·(*GetObject).neoMsgDecode(SB), NOSPLIT, $8-56 // zproto-marshal.go:1914   │TEXT ·(*GetObject).neoMsgDecode(SB), NOSPLIT, $0-56 // zproto-marshal.go:1914
        // SUBQ    $8, SP                                                       │        NO_LOCAL_POINTERS
        // MOVQ    BP, (SP) (BP save)                                           │        // FUNCDATA $0, gclocals·846769608458630ae82546dab39e913e(SB) (args)
        // LEAQ    (SP), BP (BP init)                                           │        // FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB) (no locals)
        // FUNCDATA $0, gclocals·21e863e2261befa92f8534560680bbb6(SB) (args)    │        MOVQ       data+24(SP), AX
        FUNCDATA   $1, gclocals·69c1753bd5f81501d95132d08af04464(SB) (locals)   │        CMPQ       AX, $24                        // zproto-marshal.go:1915
        MOVQ       data+32(SP), AX                                              │        JGE        pc45
        CMPQ       AX, $24                      // zproto-marshal.go:1915       │        MOVQ       ·ErrDecodeOverflow+8(SB), AX  // zproto-marshal.go:1924
        JLT        pc163                                                        │        MOVQ       ·ErrDecodeOverflow(SB), CX
        MOVQ       data+24(SP), CX                                              │        MOVQ       $0, _r1+40(SP)
        MOVQ       (CX), DX                     // zproto-marshal.go:1918       │        MOVQ       CX, _r2+48(SP)
        BSWAPQ     DX                                                           │        MOVQ       AX, _r2+56(SP)
        MOVQ       p+16(SP), BX                                                 │        RET
        MOVQ       DX, (BX)                                                     │pc45:
        LEAQ       -8(AX), DX                   // zproto-marshal.go:1919       │        MOVQ       data+16(SP), AX
        MOVQ       data+40(SP), SI                                              │        MOVQ       (AX), CX                       // zproto-marshal.go:1918
        LEAQ       -8(SI), DI                                                   │        BSWAPQ     CX
        NEGQ       DI                                                           │        MOVQ       p+8(SP), DX
        SARQ       $63, DI                                                      │        MOVQ       CX, (DX)
        ANDQ       $8, DI                                                       │        MOVQ       8(AX), CX                      // zproto-marshal.go:1919
        CMPQ       DX, $7                                                       │        BSWAPQ     CX
        JLS        pc212                                                        │        MOVQ       CX, 8(DX)
        MOVQ       (CX)(DI*1), DX                                               │        MOVQ       16(AX), AX                     // zproto-marshal.go:1920
        BSWAPQ     DX                                                           │        BSWAPQ     AX
        MOVQ       DX, 8(BX)                                                    │        MOVQ       AX, 16(DX)
        LEAQ       -16(SI), DX                  // zproto-marshal.go:1920       │        MOVQ       $24, _r1+40(SP)                // zproto-marshal.go:1921
        NEGQ       DX                                                           │        MOVQ       $0, _r2+48(SP)
        ADDQ       $-16, AX                                                     │        MOVQ       $0, _r2+56(SP)
        SARQ       $63, DX                                                      │        RET
        ANDQ       $16, DX                                                      │
        CMPQ       AX, $7                                                       │
        JLS        pc205                                                        │
        MOVQ       (CX)(DX*1), AX                                               │
        BSWAPQ     AX                                                           │
        MOVQ       AX, 16(BX)                                                   │
        MOVQ       $24, _r1+48(SP)              // zproto-marshal.go:1921       │
        MOVQ       $0, _r2+56(SP)                                               │
        MOVQ       $0, _r2+64(SP)                                               │
        // MOVQ    (SP), BP (BP restore)                                        │
        // ADDQ    $8, SP (SP restore)                                          │
        RET                                                                     │
pc163:                                                                          │
        MOVQ       ·ErrDecodeOverflow(SB), AX  // zproto-marshal.go:1924        │
        MOVQ       ·ErrDecodeOverflow+8(SB), CX                                 │
        MOVQ       $0, _r1+48(SP)                                               │
        MOVQ       AX, _r2+56(SP)                                               │
        MOVQ       CX, _r2+64(SP)                                               │
        // MOVQ    (SP), BP (BP restore)                                        │
        // ADDQ    $8, SP (SP restore)                                          │
        RET                                                                     │
pc205:                                                                          │
        PCDATA     $0, $1                       // zproto-marshal.go:1920       │
        CALL       runtime.panicindex(SB)                                       │
        UNDEF                                                                   │
pc212:                                                                          │
        PCDATA     $0, $1                       // zproto-marshal.go:1919       │
        CALL       runtime.panicindex(SB)                                       │
        UNDEF                                                                   │

Improves signal/noise ratio in generated decoder.

For example a list is encoded as

	l        u32
	[l]item  itemType

on decode len is read from data stream and for rest of data len(data) is
checked to be < l*sizeof(item).

However since l is u32 and sizeof(item) is just number the result of `l
* sizeof(item)` has also u32 type. However it could overflow e.g. for

	l		= 0x20000000
	sizeof(item)	= 8

with the l*sizeof(item) being = u32(0)	(exactly zero) -> oops.

Avoid the problem by doing all checking arithmetics with u64 ints.

* origin/master:
  client: kill .supportsTransactionalUndo()
  client: for read accesses, pick a random good node, connected or not
  storage: optimize storage layout of raw data for replication
  sqlite: remove useless AUTOINCREMENT for data.id (reuse of deleted ids is fine)
  storage: speed up reads by indexing 'obj' primarily by 'oid' (instead of 'tid')
  storage: pass schema of tables to migration methods
  storage: update backend version between each migration step

Usage of supportsTransactionalUndo() was removed from ZODB in 2007 - see
e.g. the following commits:

https://github.com/zopefoundation/ZODB/commit/a06bfc03
https://github.com/zopefoundation/ZODB/commit/e667b022
https://github.com/zopefoundation/ZODB/commit/f595f7e7
...

/reviewed-by @vpelletier
/reviewed-on nexedi/neoppod!8

* y/go: (25 commits)
  go/zodb/zodbtools: TODO (cmp, analyze)
  go/zodb/zodbtools: Catobj
  go/zodb/zodbtools: Info
  go/zodb/zodbtools: Dump
  go/zodb: Start of zodbtools - tools for managing ZODB databases
  go/zodb/fs1tools: Notes about other possible useful commands currently being there on ZODB/py side
  go/zodb/fs1tools: Reindex, Verify-index
  go/zodb/fs1tools: Dump
  go/zodb/fs1: Start fs1tools - tools for managing and maintaining ZODB FileStorage v1 databases
  go/zodb/fs1: My notes on I/O
  go/zodb/fs1: Register FileStorage to zodb & wks
  go/zodb/fs1: Actual FileStorage ZODB driver
  go/zodb/fs1: Add routines to (re)build and verify index from/wrt original FileStorage data
  go/zodb/fs1: Index save/load
  go/zodb/fs1: BTree specialized with KEY=zodb.Oid, VALUE=int64
  go/zodb: Start of FileStorage support
  go/zodb: Way for storage-drivers to be registered and for clients to open them by URL
  zodb/go: In-RAM client cache
  go/zodb: Minimal serialization compatibility with ZODB/py
  go/zodb: Tid connection with time
  ...

`zodb catobj` command to dump content of an object - similarly to `git
cat-file`. Two modes: raw and verbose with `zodb dump` like headers for
the object present.

There is no such command currently in zodbtools/py.

Command to print general information about a ZODB database.
Same as `zodb info` in zodbtools/py.

Add `zodb dump` command to dump arbitrary ZODB database in generic
format. The actual dump protocol being used here is the same as in
zodbtools/py with

	https://lab.nexedi.com/zodbtools/merge_requests/3

applied. (the MR there is OK and is just waiting for upstream ZODB to
negotiate a way to retrieve transaction extension data in raw form).

Add zodbtools which is generic (contrast to fs1tools) set of ZODB
managing utilities. Only package and command infrastructure here -
actual commands will follow up in the next patches.

Add commands for FileStorage index maintainance: manually rebuild the
index and to performe index verification.

Add various FileStorage-specific dump commands with output being
bit-to-bit exact with the following ZODB/py FileStorage tools:

- fsdump.py
- fsdump.py (verbose dumper)
- fstail.py

Please see the patch for links about this dump formats.

Build FileStorage ZODB driver out of format record loading/decoding
and index routines we just added in previous patches.

The driver supports only read-only mode so far.

Promised tests for data format interoperability with ZODB/py are added.

Build index type on top of fsb.Tree introduced in the previous patch and
add routines to save and load it to/from disk.

We ensure ZODB/py compatibility via generating test FileStorage database
+ its index and checking we can load index from it and also that if we
save an index ZODB/py can load it back. FileStorage index is hard to get
bit-to-bit identical since this index uses python pickles which can
encode the same objects in several different ways.

FileStorage index maps oid to file position storing latest data record
for this oid. This index is naturally to implement via BTree as e.g.
ZODB/py does.

In Go world there is github.com/cznic/b BTree library but without
specialization and working via interface{} it is slower than it could be
and allocates a lot. So generate specialized version of that code with
key and value types exactly suitable for FileStorage indexing.

We use a bit patched b version with speed ups for bulk-loading data via
regular point-ingestion BTree entry point:

	https://lab.nexedi.com/kirr/b x/refill

The patches has not been upstreamed because it slows down general case a
bit (only a bit, but still this is a "no" to me), and because with
dedicated bulk-loading API it could be possible to still load data
several times faster. Still current version is enough for not very-huge
indices.

Btw ZODB/py does the same (see fsBucket + friends).

Start implementing FileStorage support by adding code to load/decode
FileStorage records and way to iterate a FileStorage.

Tests will come in a later patch together with ZODB-level loading
support.

Storage drivers can register themselves via zodb.RegisterDriver.

Later cliens can request to open a storage by URL via zodb.OpenStorage.
The opener will lookup driver registry and wrap created driver instance
with common layer with cache etc to turn an IStorageDriver into fully
working IStorage.

The cache is needed so that we can provide IStorage.Prefetch
functionality generally wrapped on top of a storage driver: when an
object is loaded, the loading itself consists of steps:

1. start loading object into cache,
2. wait for the loading to complete.

This way Prefetch is naturally only "1" - start loading object into
cache but do not wait for the loading to be complete. Go's goroutines
naturally help here where we can spawn every such loading into its own
goroutine instead of explicitly programming loading in terms of a state
machine.

Since this cache is mainly needed for Prefetch to work, not to actually
cache data (though it works as cache for repeating access too), the goal
when writing it was to add minimal overhead for "data-not-yet-in-cache"
case. Current state we are not completely there yet but the latency is
acceptable - depending on the workload the cache layer adds ~

	0.5 - 1 - 3µs

to loading times.

ZODB/py serializes data using python pickles. Basically every serialized
object has two parts: class description and object state. Here we
start by providing minimal functionality to extract class-name from
serialized data.

The library used for pickle decoding (and in later patches encoding) is

	github.com/kisielk/og-rek

It was audited by me for security flaws to some extent.

Contrary to Python pickle module it does not run arbitrary code on
decoding.

Since in ZODB TIDs are corresponding to time, provide functionality to
convert a tid to timestamp. Do so in exactly the same way as ZODB/py
does for interoperability.