'Get the state dict corresponding to a particular event
Args:
event_id(str): event whose state should be returned
types(list[(str, str)]|None): List of (type, state_key) tuples
which are used to filter the state fetched. May be None, which
matches any key
Returns:
A deferred dict from (type, state_key) -> state_event'

@defer.inlineCallbacks
def get_state_for_event(self, event_id, types=None):

    state_map = (yield self.get_state_for_events([event_id], types))
    defer.returnValue(state_map[event_id])
