Found by pylint.

This fixes the following error:

PACKET #0x0012 AnswerTruncate                 > M1 (127.0.0.3:1)
ERROR  Pre-mortem data:
ERROR  Traceback (most recent call last):
ERROR    File "neo/storage/app.py", line 166, in run
ERROR      self._run()
ERROR    File "neo/storage/app.py", line 207, in _run
ERROR      self.doOperation()
ERROR    File "neo/storage/app.py", line 308, in doOperation
ERROR      task_queue[-1].next()
ERROR    File "neo/storage/handlers/storage.py", line 240, in push
ERROR      conn.notify(Packets.AddObject(oid, serial, *object[2:]))
ERROR  TypeError: 'bool' object is unsubscriptable

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

Without this, transactions and even objects may be missing on the backup
database.

This fixes the following assertion failure:

PACKET #0x021d Ping                           > M1 (10.0.114.64:14001)
INFO   timeout for #0x0000021d with <ClientConnection(uuid=M1, address=10.0.114.64:14001, closed=0, handler=BackupHandler) at 2955590>
DEBUG  connection closed for <ClientConnection(uuid=M1, address=10.0.114.64:14001, closed=1, handler=BackupHandler) at 2955590>
ERROR  upstream master is down: connection lost
ERROR  Pre-mortem data:
ERROR  Traceback (most recent call last):
ERROR    File "neo/master/app.py", line 134, in run
ERROR      self._run()
ERROR    File "neo/master/app.py", line 154, in _run
ERROR      self.playPrimaryRole()
ERROR    File "neo/master/app.py", line 344, in playPrimaryRole
ERROR      self.backup_app.provideService()
ERROR    File "neo/master/backup_app.py", line 129, in provideService
ERROR      conn.close()
ERROR    File "neo/lib/connection.py", line 541, in close
ERROR      assert not self.isPending()
ERROR  AssertionError

SQL tables can be upgraded using:
  UPDATE config SET name = 'nid' WHERE name = 'uuid';

then for MySQL:
  ALTER TABLE pt CHANGE uuid nid INT NOT NULL;

or SQLite:
  ALTER TABLE pt RENAME TO old_pt;
  CREATE TABLE pt (rid INTEGER NOT NULL, nid INTEGER NOT NULL, state INTEGER NOT NULL, PRIMARY KEY (rid, nid));
  INSERT INTO pt SELECT * from old_pt;
  DROP TABLE old_pt;

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

And contrary to previous implementation, there is no automatic reallocation
of partition amongst remaining nodes. Now only 'tweak' does that.

This fixes a bug in backup mode where 'loid' was not updated.

This changes is similar to commit 3d0fab20
("logger: retry instead of raise if the log is locked by another process").

This changes completely how to get data from storages than is not too
recent and NEO now behaves as expected by ZODB, instead of trying to
snapshot at Storage level.
However, ZODB should probably be changed to avoid double loading when
an invalidation is received during a load.