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.