Split out DB writes in federation handler

This will allow us to easily add an internal replication API to proxy
these reqeusts to master, so that we can move federation APIs to
workers.

synapse/storage/events.py

@@ -231,12 +231,18 @@ class EventsStore(EventsWorkerStore):
 
         self._state_resolution_handler = hs.get_state_resolution_handler()
 
+    @defer.inlineCallbacks
     def persist_events(self, events_and_contexts, backfilled=False):
         """
         Write events to the database
         Args:
             events_and_contexts: list of tuples of (event, context)
-            backfilled: ?
+            backfilled (bool): Whether the results are retrieved from federation
+                via backfill or not. Used to determine if they're "new" events
+                which might update the current state etc.
+
+        Returns:
+            Deferred[int]: he stream ordering of the latest persisted event
         """
         partitioned = {}
         for event, ctx in events_and_contexts:
@@ -253,10 +259,14 @@ class EventsStore(EventsWorkerStore):
         for room_id in partitioned:
             self._maybe_start_persisting(room_id)
 
-        return make_deferred_yieldable(
+        yield make_deferred_yieldable(
             defer.gatherResults(deferreds, consumeErrors=True)
         )
 
+        max_persisted_id = yield self._stream_id_gen.get_current_token()
+
+        defer.returnValue(max_persisted_id)
+
     @defer.inlineCallbacks
     @log_function
     def persist_event(self, event, context, backfilled=False):