This fixes the following crash (for example when a client disconnects during
tpc_finish):

Traceback (most recent call last):
  ...
  File "neo/master/handlers/storage.py", line 68, in answerInformationLocked
    self.app.tm.lock(ttid, conn.getUUID())
  File "neo/master/transactions.py", line 338, in lock
    if self._ttid_dict[ttid].lock(uuid) and self._queue[0][1] == ttid:
IndexError: list index out of range

Before this change, a storage node did 3 commits per transaction:
- once all data are stored
- when locking the transaction
- when unlocking the transaction

The last one is not important for ACID. In case of a crash, the transaction
is unlocked again (verification phase). By deferring it by 1 second, we
only have 2 commits per transaction during high activity because all pending
changes are merged with the commits caused by other transactions.

This change compensates the extra commit(s) per transaction that were
introduced in commit 7eb7cf1b
("Minimize the amount of work during tpc_finish").

Since commit d2d77437 ("client: make the cache
tolerant to late invalidations when the entry is in the history queue"),
invalidated items became current again when they were moved to the history
queue, which was wrong for 2 reasons:
- only the last items of _oid_dict values may have next_tid=None,
- and for such items, they could be wrongly reused when caching the real
  current data.

This fixes the following scenario:
1. the master sends invalidations to clients,
   and unlocks to storages  (oid1, tid1)
2. the storage receives/processes the unlock
3. the client asks data (oid1, tid0)
4. the storage returns tid1 as next tid, whereas it's still None in the cache
   (before, it caused an assertion failure)
6. the client processes invalidations

See commit c277ed20
("client: really process all invalidations in poll thread").

This is a partial implementation. To truncate at a smaller tid, you must wait
that data is imported up to this tid and stop using the Importer backend.

This backend does not support replication. Even if we implemented it, such node
could only be a source for other nodes so we should never delete transactions.

This fixes a regression caused by
commit eef52c27