Make presence.get_new_events a bit faster

We do this by caching the set of users a user shares rooms with.

synapse/storage/roommember.py

@@ -280,6 +280,22 @@ class RoomMemberStore(SQLBaseStore):
             user_id, membership_list=[Membership.JOIN],
         )
 
+    @cachedInlineCallbacks(max_entries=50000, cache_context=True, iterable=True)
+    def get_users_who_share_room_with_user(self, user_id, cache_context):
+        rooms = yield self.get_rooms_for_user(
+            user_id, on_invalidate=cache_context.invalidate,
+        )
+
+        user_who_share_room = set()
+        for room in rooms:
+            user_ids = yield self.get_users_in_room(
+                room.room_id, on_invalidate=cache_context.invalidate,
+            )
+            logger.info("Users in room: %r %r", room.room_id, user_ids)
+            user_who_share_room.update(user_ids)
+
+        defer.returnValue(user_who_share_room)
+
     def forget(self, user_id, room_id):
         """Indicate that user_id wishes to discard history for room_id."""
         def f(txn):