'RequestQueue._clear_possible_locks' is not handled correctly in event_manager

[Mantisus]

When the Event.MIGRATING event occurs, we get an error

RuntimeWarning: coroutine 'RequestQueue._clear_possible_locks' was never awaited
  handle = None # Needed to break cycles when an exception occurs.
RuntimeWarning: Enable tracemalloc to get the object allocation traceback

Due to incorrect listener initialization

event_manager.on(event=Event.MIGRATING, listener=lambda _: self._clear_possible_locks())
event_manager.on(event=Event.ABORTING, listener=lambda _: self._clear_possible_locks())

This fix is worth doing after #800, since _clear_possible_locks doesn't expect any parameters