Fix incredubly slow back pagination query

If a client didn't specify a from token when paginating backwards
synapse would attempt to query the (global) maximum topological token.
This a) doesn't make much sense since they're room specific and b) there
are no indices that lets postgres do this efficiently.

synapse/storage/stream.py

@@ -521,13 +521,20 @@ class StreamStore(SQLBaseStore):
         )
 
     @defer.inlineCallbacks
-    def get_room_events_max_id(self, direction='f'):
+    def get_room_events_max_id(self, room_id=None):
+        """Returns the current token for rooms stream.
+
+        By default, it returns the current global stream token. Specifying a
+        `room_id` causes it to return the current room specific topological
+        token.
+        """
         token = yield self._stream_id_gen.get_current_token()
-        if direction != 'b':
+        if room_id is None:
             defer.returnValue("s%d" % (token,))
         else:
             topo = yield self.runInteraction(
-                "_get_max_topological_txn", self._get_max_topological_txn
+                "_get_max_topological_txn", self._get_max_topological_txn,
+                room_id,
             )
             defer.returnValue("t%d-%d" % (topo, token))
 
@@ -579,11 +586,11 @@ class StreamStore(SQLBaseStore):
             lambda r: r[0][0] if r else 0
         )
 
-    def _get_max_topological_txn(self, txn):
+    def _get_max_topological_txn(self, txn, room_id):
         txn.execute(
             "SELECT MAX(topological_ordering) FROM events"
-            " WHERE outlier = ?",
-            (False,)
+            " WHERE room_id = ?",
+            (room_id,)
         )
 
         rows = txn.fetchall()