I made a mistake in commit 13a64cfe
("Simplify definition of packets by computing automatically their codes").
My intention was that the code an answer packet continues to only differ by the
highest bit, as implemented now by this commit.

Before:
  0x0001, 0x8002   Ask1, Answer1
  0x0003           Notify2
  0x0004, 0x8005   Ask3, Answer3
  0x0006, 0x8007   Ask4, Answer4

After:
  0x0001, 0x8001   Ask1, Answer1
  0x0002           Notify2
  0x0003, 0x8003   Ask3, Answer3
  0x0004, 0x8004   Ask4, Answer4

This makes the protocol easier to document.

And by not wasting the range of possible values, it seems we have enough
space to shrink to a single byte.

This also removes code that became meaningless since that codes are generated
automatically.

Although data that are already transferred aren't transferred again, checking
that the data are there for a whole partition can still be a lot of work for
big databases. This commit is a major performance improvement in that a storage
node that gets disconnected for a short time now gets fully operational quite
instantaneously because it only has to replicate the new data. Before, the time
to recover depended on the size of the DB.

For OUT_OF_DATE cells, the difficult part was that they are writable and
can then contain holes, so we can't just take the last TID in trans/obj
(we wrongly did that at the beginning, and then committed
6b1f198f as a workaround). We solve that
by storing up to where it was up-to-date: this value is initialized from
the last TIDs in trans/obj when the state switches from UP_TO_DATE/FEEDING.

There's actually one such OUT_OF_DATE TID per assigned cell (backends store
these values in the 'pt' table). Otherwise, a cell that still has a lot to
replicate would still cause all other cells to resume from the a very small
TID, or even ZERO_TID; the worse case is when a new cell is assigned to a node
(as a result of tweak).

For UP_TO_DATE cells of a backup cluster, replication was resumed from the
maximum TID at which all assigned cells are known to be fully replicated.
Like for OUT_OF_DATE cells, the presence of a late cell could cause a lot of
extra work for others, the worst case being when setting up a backup cluster
(it always restarted from ZERO_TID as long as at least 1 cell was still empty).
Because UP_TO_DATE cells are guaranteed to have no holes, there's no need to
store extra information: we simply look at the last TIDs in trans/obj.
We even handle trans & obj independently, to minimize the work in 1 table
(i.e. trans since it's processed first) if the other is late (obj).

There's a small change in the protocol so that OUT_OF_DATE enum value equals 0.
This way, backends can store the OUT_OF_DATE TID (verbatim) in the same column
as the cell state.

Note about MySQL changes in commit ca58ccd7:
what we did as a workaround is not one any more. Now, we do so much on Python
side that it's unlikely we could reduce the number of queries using GROUP BY.
We even stopped doing that for SQLite.

It was confusing and there's already the 'Unlock TXN' log just before abort()
is called (in this case, it's more a cleanup than an abort).

Future migration steps are likely to alter tables, possibly with
transformation of data, and this is complicated for both supported backend.

Some changes in the storage format are minor and applying them automatically
would cost too much for big databases.

Here, we apply them manually so that testStorageUpgrade will be able to
compare dumps.

We hope however that with improvements like
  https://jira.mariadb.org/browse/MDEV-12836
we'll be able to implement more migration steps
and revert parts of this commit.

These dumps were generated with an old version of NEO, plus a backport of the
test that will use them.

In MySQL dumps, --hex-blob was used only for inserts in the 'data' table.

- for FileStorage DB, make sure a transaction index is built at most once
- for other DB types, reopen the DB in the subprocess

Now that we have specific code for FileStorage, the generic case is not tested
anymore. We should add a test using ZEO. Or better, and in some way crazy,
one with NEO, but one would need to fix a special case in getObject.

The protocol version is increased to ensure that client nodes are able to
handle an empty 'extension' field in AnswerTransactionInformation.

It also means that once new transactions are written, going back to a previous
revision is not possible.

The correct way to specify a start/stop tid is when constructing the 'source'
object, hence the remove of start/stop args. In fact, source.iterator()
does not always take such args.

On the other hand, when resuming import, Application.importFrom must manage
with incomplete preindex.

Same as previous commit: only cosmetics so optional.

'title' means both process name and command line.

This is cosmetics so it won't fail if the 'setproctitle' module
is not available.

A new subprocess is used to:
- fetch data from the source DB
- repickle to change oids (when merging several DB)
- compress
- checksum

This is mostly useful for the second step, which is relatively much slower than
any other step, while not releasing the GIL.

By using a second CPU core, it is also often possible to use a better
compression algorithm for free (e.g. zlib=9). Actually, smaller data can speed
up the writing process.

In addition to greatly speed up the import by parallelizing fetch+process with
write, it also makes the main process more reactive to queries from client
nodes.

By doing the work with secondary connections to the underlying databases,
asynchronously and in a separate process, this should have minimal impact on
the performance of the storage node. Extra complexity comes from backends that
may lose connection to the database (here MySQL): this commit fully implements
reconnection.

For FileStorage DB, this avoids:
- keeping a lock on the source DB during the whole import,
- saving the whole index when the import was resumed.

It was disabled by mistake in commit fd80cc30.

- Stop using NEO source code as sample data.
- For ZODB5, add a test that does not merge several DB.

In the Importer storage backend, the repickler code never really worked with
ZODB 5 (use of protocol > 1), and now the test does not pass anymore.

The other issues caused by ZODB commit 12ee41c47310156027a674932df34b60de86ba36
are fixed:

  TypeError: list indices must be integers, not binary

  ValueError: unsupported pickle protocol: 3

Although not necessary as long as we don't support Python 3,
this commit also replaces `str` by `bytes` in a few places.

When importing a FileStorage DB without interruption and without having to
serve client nodes, the index built by speedupFileStorageTxnLookup is useless.
Such case happens when doing simulation tests and on DB with many oids,
it can take a lot of time and memory for nothing.

This was forgotten in commit 5de0ff3a.

The Importer storage backend already does this.

This fixes a random failure in testSafeTweak:

  failureException: 'UU.|U.U|.UU' != 'UU.|.UU|U.U'

This is a follow-up of commit 2ca7c335,
which changed 'tweak' not to discard readable cells too quickly.

The scenario of a storage being lost whereas it has feeding cells was forgotten.
These must be discarded immediately, otherwise we end up with more up-to-date
cells than wanted. Without the change in outdate(), testSafeTweak would end
with: UU.|U.U|UUU

Once replication is optimized not to always restart checking cells from the
beginning:
- Remembering that an out-of-date cell was feeding could be a safer
  option, but it may not be worth the extra complexity.
- Another possibility may be to replace the FEEDING state by an automatic
  partial tweak that only discards up-to-date cells too many whenever a cell
  becomes up-to-date.