Given an actor unique id return the actor proxy.

def get_actor(self, actor, aid, check_monitor=True):
        '''Given an actor unique id return the actor proxy.'''
        a = super().get_actor(actor, aid)
        if a is None:
            if aid in self.monitors:  # Check in monitors aid
                return self.monitors[aid]
            elif aid in self.managed_actors:
                return self.managed_actors[aid]
            elif aid in self.registered:
                return self.registered[aid]
            else:  # Finally check in workers in monitors
                for m in self.monitors.values():
                    a = m.get_actor(aid, check_monitor=False)
                    if a is not None:
                        return a
        else:
            return a