Cap the time to retry txns to appservices to 8.5 minutes

There's been numerous issues with people playing around with their
application service and then not receiving events from their HS for
ages due to backoff timers reaching crazy heights (albeit capped at
< 1 day).

Reduce the max time between pokes to be 8.5 minutes (2^9 secs) which
is quick enough for people to wait it out (avg wait time being 4.25 min)
but long enough to actually give the AS breathing room if it needs it.

synapse/appservice/scheduler.py

@@ -224,8 +224,8 @@ def recover(self):
         self.clock.call_later((2 ** self.backoff_counter), self.retry)
 
     def _backoff(self):
-        # cap the backoff to be around 18h => (2^16) = 65536 secs
-        if self.backoff_counter < 16:
+        # cap the backoff to be around 8.5min => (2^9) = 512 secs
+        if self.backoff_counter < 9:
             self.backoff_counter += 1
         self.recover()