Fix instantiation of message retention purge jobs

When figuring out which topological token to start a purge job at, we
need to do the following:

1. Figure out a timestamp before which events will be purged
2. Select the first stream ordering after that timestamp
3. Select info about the first event after that stream ordering
4. Build a topological token from that info

In some situations (e.g. quiet rooms with a short max_lifetime), there
might not be an event after the stream ordering at step 3, therefore we
abort the purge with the error `No event found`. To mitigate that, this
patch fetches the first event _before_ the stream ordering, instead of
after.

synapse/handlers/pagination.py

@@ -156,7 +156,7 @@ class PaginationHandler(object):
 
             stream_ordering = yield self.store.find_first_stream_ordering_after_ts(ts)
 
-            r = yield self.store.get_room_event_after_stream_ordering(
+            r = yield self.store.get_room_event_before_stream_ordering(
                 room_id, stream_ordering,
             )
             if not r: