Fix bugs in the /keys/changes api

* `get_forward_extremeties_for_room` takes a numeric `stream_ordering`. We were
  passing a `RoomStreamToken`, which meant that it returned the *current*
  extremities, rather than those corresponding to the `from_token`. However:
* `get_state_ids_for_events` required a second ('types') parameter; this meant
  that a `TypeError` was thrown and we ended up acting as though there was *no*
  prev state.
* `get_state_ids_for_events` actually returns a map from event_id to state
  dictionary - just looking up the state keys in it again meant that we acted
  as though there was no prev state. We now check if each member's state has
  changed since *any* of the extremities.

Also add/fix some comments.

synapse/storage/event_federation.py

@@ -281,15 +281,30 @@ class EventFederationStore(SQLBaseStore):
         )
 
     def get_forward_extremeties_for_room(self, room_id, stream_ordering):
+        """For a given room_id and stream_ordering, return the forward
+        extremeties of the room at that point in "time".
+
+        Throws a StoreError if we have since purged the index for
+        stream_orderings from that point.
+
+        Args:
+            room_id (str):
+            stream_ordering (int):
+
+        Returns:
+            deferred, which resolves to a list of event_ids
+        """
         # We want to make the cache more effective, so we clamp to the last
         # change before the given ordering.
         last_change = self._events_stream_cache.get_max_pos_of_last_change(room_id)
 
         # We don't always have a full stream_to_exterm_id table, e.g. after
         # the upgrade that introduced it, so we make sure we never ask for a
-        # try and pin to a stream_ordering from before a restart
+        # stream_ordering from before a restart
         last_change = max(self._stream_order_on_start, last_change)
 
+        # provided the last_change is recent enough, we now clamp the requested
+        # stream_ordering to it.
         if last_change > self.stream_ordering_month_ago:
             stream_ordering = min(last_change, stream_ordering)