NEO did not ensure that all data and metadata are written on disk before
tpc_finish, and it was for example vulnerable to ENOSPC errors.
In other words, some work had to be moved to tpc_vote:

- In tpc_vote, all involved storage nodes are now asked to write all metadata
  to ttrans/tobj and _commit_. Because the final tid is not known yet, the tid
  column of ttrans and tobj now contains NULL and the ttid respectively.

- In tpc_finish, AskLockInformation is still required for read locking,
  ttrans.tid is updated with the final value and this change is _committed_.

- The verification phase is greatly simplified, more reliable and faster. For
  all voted transactions, we can know if a tpc_finish was started by getting
  the final tid from the ttid, either from ttrans or from trans. And we know
  that such transactions can't be partial so we don't need to check oids.

So in addition to minimizing the risk of failures during tpc_finish, we also
fix a bug causing the verification phase to discard transactions with objects
for which readCurrent was called.

On performance side:

- Although tpc_vote now asks all involved storages, instead of only those
  storing the transaction metadata, the client has been improved to do this
  in parallel. The additional commits are also all done in parallel.

- A possible improvement to compensate the additional commits is to delay the
  commit done by the unlock.

- By minimizing the time to lock transactions, objects are read-locked for a
  much shorter period. This is even more important that locked transactions
  must be unlocked in the same order.

Transactions with too many modified objects will now timeout inside tpc_vote
instead of tpc_finish. Of course, such transactions may still cause other
transaction to timeout in tpc_finish.

We can never receive several answers from the same node.

testVerification is dropped for the same reason as for testEvent and most of
testConnection, since there is much incoming changes for verification.

- Last known TID was not updated when recovering a transaction.
- Missing OIDs were ignored, which caused partial transactions to be committed
  instead of being deleted.

This fixes invalid next_serial entries in cache,
and the following error for values not in cache:

  Traceback (most recent call last):
    File "ZODB/Connection.py", line 856, in setstate
      self._setstate(obj)
    File "ZODB/Connection.py", line 894, in _setstate
      self._load_before_or_conflict(obj)
    File "ZODB/Connection.py", line 922, in _load_before_or_conflict
      if not self._setstate_noncurrent(obj):
    File "ZODB/Connection.py", line 945, in _setstate_noncurrent
      assert end is not None
  AssertionError

Application will hold SSL parameters.

This happened between storage nodes of different clusters because they're not
informed about their state, e.g. a dead upstream storage node.

In any case, logs were flooded at 100% cpu usage.

Of course, neoctl should only connect to the admin, but in case of user mistake
the queried node should not die. Ideally, all other commands should be checked.

Also, drop extraneous parentheses in another set's creation.

In particular, do not flush all debug logs still in memory

- catch OperationFailure
- reset transaction manager when leaving backup mode
- send appropriate target tid to a storage that updates a outdated cell
- clean up partition table when leaving BACKINGUP state unexpectedly
- make sure all readable cells of a partition have the same 'backup_tid'
  if they have the same data, so that we know when internal replication is
  finished when leaving backup mode
- fix storage not finished internal replication when leaving backup mode

This fixes a regression in commit ad01f379
("fix leaving backup mode, including truncating to consistent TID")

- fix stopping backup cluster
- fix leaving backup mode, including truncating to consistent TID
- fix backup_tid on master and storages