Only get cached state from context in persist_event

We don't want to bother pulling out the current state from the DB since
until we know we have to. Checking the context for state is just an
optimisation.

synapse/events/snapshot.py

@@ -163,6 +163,9 @@ class EventContext(object):
         context._prev_state_id = input["prev_state_id"]
         context._event_type = input["event_type"]
         context._event_state_key = input["event_state_key"]
+
+        context._current_state_ids = None
+        context._prev_state_ids = None
         context._fetching_state_deferred = None
 
         context.state_group = input["state_group"]
@@ -214,6 +217,16 @@ class EventContext(object):
 
         defer.returnValue(self._prev_state_ids)
 
+    def get_cached_current_state_ids(self):
+        """Gets the current state IDs if we have them already cached.
+
+        Returns:
+            dict[(str, str), str]|None: Returns None if state_group
+            is None, which happens when the associated event is an outlier.
+        """
+
+        return self._current_state_ids
+
     @defer.inlineCallbacks
     def _fill_out_state(self, store):
         """Called to populate the _current_state_ids and _prev_state_ids