master,client: ignore notifications before complete initialization

A backup master crashed with the following traceback after a reconnection:

    Traceback (most recent call last):
      File "neo/master/app.py", line 127, in run
        self._run()
      File "neo/master/app.py", line 147, in _run
        self.playPrimaryRole()
      File "neo/master/app.py", line 348, in playPrimaryRole
        self.backup_app.provideService())
      File "neo/master/backup_app.py", line 123, in provideService
        poll(1)
      File "neo/lib/event.py", line 126, in poll
        to_process.process()
      File "neo/lib/connection.py", line 500, in process
        self._handlers.handle(self, self._queue.pop(0))
      File "neo/lib/connection.py", line 110, in handle
        self._handle(connection, packet)
      File "neo/lib/connection.py", line 125, in _handle
        handler.packetReceived(connection, packet)
      File "neo/lib/handler.py", line 117, in packetReceived
        self.dispatch(*args)
      File "neo/lib/handler.py", line 66, in dispatch
        method(conn, *args, **kw)
      File "neo/master/handlers/backup.py", line 52, in invalidateObjects
        app.invalidatePartitions(tid, partition_set)
      File "neo/master/backup_app.py", line 257, in invalidatePartitions
        self.triggerBackup(node)
      File "neo/master/backup_app.py", line 281, in triggerBackup
        assert cell_list, offset
    AssertionError: 0