This is intended as a sanity check, so simple typos in neoctl truncate
command do not easily lead to the entire database being wiped.

Before this change, the only distinction between a timestamp and a TID was
the presence of the decimal separator, ".". As a result, a timestamp
mistakenly provided without a decimal separator would be interpreted as a
TID, which will be somewhere in January 1900 (as TIDs are 64bits with much
finer accuracy than timestamps). When used to truncate a database, and in
the absence of sanity checks, this would simply wipe the database.

So, instead of just relying on a decimal separator, require a longer
string. Make it a prefix for readability. Also, TIDs are more niche than
timestamp, require them to have a mark, and do not require anything from
timestamps.

This will be used by an external GC.

To be pushed upstream.

set_wakeup_fd only works in main thread.

See commit f47dd646.

Since the switch to msgpack, there's no more type checking at protocol level
and for example passing an integer would cause storage nodes to crash.

But as shown here, the type checking of the old protocol was not always
enough, because data structures at client side could anyway get wrong.

Else it leads to DB corruption and a crash of the master.

This fixes commit 0e43dd1f
("Fix signals not always being processed as soon as possible").

See commit b6f821a2.

Julien notes this is very likely unneeded:
!21 (diffs, comment 195929)

We had it like this since 01a01c8c (client: Add support for zodburi),
but I rechecked zodburi codebase now and it does not do any similar
lowering anywhere.

So drop support for case normalization in zurl options.

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

Unfortunately after creating SSL context it is not possible, or at least
I could not find how, to retrieve original credentials with which the
context was created. However wendelin.core needs to be able to take a
client storage, reconstruct zurl to refer to that particular storage,
and pass that zurl to wcfs, so that wcfs, in turn, could access the same
ZODB database.

Given a NEO client instance, it is already possible to retrieve
master_nodes, cluster name, and detect whether SSL is being in use.
However without being able to retrieve original SSL credentials,
reconstructed zurl will not be full and wcfs won't be able to use
exactly the same secrets as python part does.

-> Help wendelin.core by remembering which ca/cert/key were used to
build SSL context.

This information is used by zstor_2zurl in wendelin.core here:

https://lab.nexedi.com/nexedi/wendelin.core/blob/885b3556/lib/zodb.py#L390-418

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

Similarly to how it is done with e.g. http:// and https:// - if neos://
is given TLS usage is forced and ca/cert/key must be there either in the
URI itself, or in $NEO_CA, $NEO_CERT and $NEO_KEY environment variables
mimicking the way how e.g. for https:// TLS credentials are taken from
host environment, not from the uri.

The latter might be usability convenience, but is also useful for WCFS
which needs to be able to remove secrets from uri on zurl normalization.

Please see discussion at !18 (comment 184439)
for details.

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

Because list of masters and cluster name must be already present in
netloc and path. Previously e.g.

	neo://db@α,β,γ?master_nodes=a,b,c"

would mean to use master nodes {a,b,c} not {α,β,γ}. Now it is treated as
invalid URI to remove ambiguity. Same for cluster name.

/cc @levin.zimmermann
/reviewed-by @jm
/reviewed-on !21

Pre-mortem data:
Traceback (most recent call last):
File "neo/master/app.py", line 172, in run
self._run()
File "neo/master/app.py", line 180, in _run
self.listening_conn = ListeningConnection(self, None, self.server)
File "neo/lib/connection.py", line 298, in __init__
connector.makeListeningConnection()
File "neo/lib/connector.py", line 133, in makeListeningConnection
self._error('listen', e)
File "neo/lib/connector.py", line 93, in _error
raise ConnectorException
ConnectorException
Traceback (most recent call last):
  File "neomaster", line 50, in <module>
    sys.exit(neo.scripts.neomaster.main())
  File "neo/scripts/neomaster.py", line 31, in main
    app.run()
  File "neo/master/app.py", line 175, in run
    self.log()
  File "neo/master/app.py", line 167, in log
    if self.pt is not None:
AttributeError: 'Application' object has no attribute 'pt'

This is still pack without garbage collection, and without deleting
any transaction metadata ('trans' table).

Partial pack means that the client can take a list of oids: only these
oids will be packed. No API is defined yet at IStorage level.

Storage nodes pack in background, independently from other storage
nodes, partition by partition, and calling IStorage.pack() returns
immediately (though internally, NEO does have a mechanism to wait
until it's done, which can be required for some ZODB unit tests).

This new implementation also introduces the concept of signing pack
orders. The idea is that calling IStorage.pack() only records a pack
order in the database, that can be reviewed/approved/rejected using
an UI that is left to be done. For the moment, pack orders are
automatically approved (by the master).

Internally, pack orders are stored as extra metadata of a transaction.
IOW, IStorage.pack() implies the commit of an (empty) transaction.

IStorage.pack() can be called without waiting for the previous one
to be completed. Pack orders processed in the same order as they are
requested:
- an unsigned pack order blocks the processing of any newer pack order;
- rejected pack order are ignored.

Approving a pack order also triggers pack on backup clusters.
That's the simplest way to have everything consistent.
Maybe later we could identify scenarios where it would be ok
to unsign pack orders during asynchronous replication.

The feature to check replicas is marked as experimental because it is
not aware of differences that can happen during pack operations.
_______________________________________________________________________

About concurrency within the storage node, a first implementation
extended what was done to delete partitions in background (see
previous commit). But here, the job can't be easily split in splices
that are never too big:
- it's simpler to never split the processing of an oid but this can
  freeze the application for a long time when packing an oid that was
  modified many times (e.g. 30 min for an oid with 20 millions
  historical records);
- then an attempt so that an oid can be processed in several times was
  inefficient, maybe due to a limit in RocksDB (packing the oid in the
  above example would take days during which NEO is significantly
  slower).

So background database jobs were moved to a separate thread, using a
separate connection to the underlying database. This is obviously
only useful for the MySQL backend. In order to share as much code as
possible between backends, SQLite also does the work in a separate
thread but sharing the main connection instead of opening a separate
one (so such backend would not be suited in the above example).

But deleting raw data with a secondary connection is not possible
without fsyncing too often (or transaction isolation issues...): these
deletions are deferred by recording them in a new table, which is
processed later with the main connection. This is not so bad because
the actual deletion of raw data is usually more efficient this way
(more sequential IO).

Here are a few numbers:
- without load: 10h45 (12h for the first reimplementation)
- with a load that normally takes 6h58:
  - load: 7h33 (so 8.4% slower)
  - pack: 15h36 (+4h51)

As explained above, the pack of a partition is split in 2 steps:
- the longest one (here 78% without load) should have negligible
  peformance impact on the application because the work is done in a
  separate thread with a secondary connection, and also with something
  to minimize GIL impact by prioritizing the main thread;
- the shortest one (22%) to process the deferred deletions,
  with even lower priority than replication: it tries to split
  the work in tasks that take ~10ms.

This is implemented using the same concurrency mechanism as for the
replication: the work is split in slices that should be small enough
to avoid slowing down network requests significantly.

undone_data_tid can't be equal to a TTID.

It has never been enabled and the code to drop partitions will be
changed in a way that only 'trans' may still benefit of partitioning.
We'll see in the future if we have cases where 'trans' is too big to
delete all rows (of a given partition) in a single query.

- When undoing current record, fix:
  - crash of storage nodes that don't have the undo data (non-readable cells);
  - and conflict resolution.
- Fix undo deduplication in replication when NEO deduplication is disabled.
- client: minor fixes in undo() about concurrent storage disconnections
  and PT updates.