This is a preliminary commit, without any functional change,
just to make the next one easier to review.

We already have logs when a connection fails,
and ask() raises ConnectionClosed if the connection is closed.

SourceForge currently has too many issues.

Obviously, oids can't be automatically invalidated if the undo is done directly
at the storage level.

In commit 9cca0f8e, only 1 bug was found.

FileStorage has been fixed in commit b7ea4e6f708dcded329332b24a9d70211a6b6368

Cache items are stored in double-linked chains. In order to quickly know the
number of history items, an extra attribute is used to count them. It was not
always decremented when a history item was removed.

This led to the following exception:
  <ClientCache history_size=100000 oid_count=1959 size=20970973 time=2849049 queue_length=[1, 7, 738, 355, 480, 66, 255, 44, 3, 5, 2, 1, 3, 4, 2, 2] (life_time=10000 max_history_size=100000 max_size=20971520)>
  poll raised, retrying
  Traceback (most recent call last):
    ...
    File "neo/client/handlers/master.py", line 137, in packetReceived
      cache.store(oid, data, tid, None)
    File "neo/client/cache.py", line 247, in store
      self._add(head)
    File "neo/client/cache.py", line 129, in _add
      self._remove(head)
    File "neo/client/cache.py", line 136, in _remove
      level = item.level
  AttributeError: 'NoneType' object has no attribute 'level'

This fixes the following issue:

WARNING replication aborted for partition 1
DEBUG   connection started for <ClientConnection(uuid=None, address=...:43776, handler=StorageOperationHandler, fd=10, on_close=onConnectionClosed, connecting, client) at 7f5d2067fdd0>
DEBUG   connect failed for <SocketConnectorIPv6 at 0x7f5d2067fe10 fileno 10 ('::', 0), opened to ('...', 43776)>: ENETUNREACH (Network is unreachable)
WARNING replication aborted for partition 5
DEBUG   connection started for <ClientConnection(uuid=None, address=...:43776, handler=StorageOperationHandler, fd=10, on_close=onConnectionClosed, connecting, client) at 7f5d1c409510>
PACKET  #0x0000 RequestIdentification          > None (...:43776)  | (<EnumItem STORAGE (1)>, None, ('...', 60533), '...')
ERROR   Pre-mortem data:
ERROR   Traceback (most recent call last):
ERROR     File "neo/storage/app.py", line 157, in run
ERROR       self._run()
ERROR     File "neo/storage/app.py", line 197, in _run
ERROR       self.doOperation()
ERROR     File "neo/storage/app.py", line 285, in doOperation
ERROR       poll()
ERROR     File "neo/storage/app.py", line 95, in _poll
ERROR       self.em.poll(1)
ERROR     File "neo/lib/event.py", line 121, in poll
ERROR       self._poll(blocking)
ERROR     File "neo/lib/event.py", line 165, in _poll
ERROR       if conn.readable():
ERROR     File "neo/lib/connection.py", line 481, in readable
ERROR       self._closure()
ERROR     File "neo/lib/connection.py", line 539, in _closure
ERROR       self.close()
ERROR     File "neo/lib/connection.py", line 531, in close
ERROR       handler.connectionClosed(self)
ERROR     File "neo/lib/handler.py", line 135, in connectionClosed
ERROR       self.connectionLost(conn, NodeStates.TEMPORARILY_DOWN)
ERROR     File "neo/storage/handlers/storage.py", line 59, in connectionLost
ERROR       replicator.abort()
ERROR     File "neo/storage/replicator.py", line 339, in abort
ERROR       self._nextPartition()
ERROR     File "neo/storage/replicator.py", line 260, in _nextPartition
ERROR       None if name else app.uuid, app.server, name or app.name))
ERROR     File "neo/lib/connection.py", line 562, in ask
ERROR       raise ConnectionClosed
ERROR   ConnectionClosed

This fixes a lock leak on storages, causing further transactions to timeout.

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.