Fix some looping_call calls which were broken in #3604

It turns out that looping_call does check the deferred returned by its
callback, and (at least in the case of client_ips), we were relying on this,
and I broke it in #3604.

Update run_as_background_process to return the deferred, and make sure we
return it to clock.looping_call.

synapse/storage/client_ips.py

@@ -102,7 +102,7 @@ class ClientIpStore(background_updates.BackgroundUpdateStore):
                 to_update,
             )
 
-        run_as_background_process(
+        return run_as_background_process(
             "update_client_ips", update,
         )
 

synapse/storage/devices.py

@@ -712,7 +712,7 @@ class DeviceStore(SQLBaseStore):
 
             logger.info("Pruned %d device list outbound pokes", txn.rowcount)
 
-        run_as_background_process(
+        return run_as_background_process(
             "prune_old_outbound_device_pokes",
             self.runInteraction,
             "_prune_old_outbound_device_pokes",

synapse/storage/event_federation.py

@@ -549,7 +549,7 @@ class EventFederationStore(EventFederationWorkerStore):
                 sql,
                 (self.stream_ordering_month_ago, self.stream_ordering_month_ago,)
             )
-        run_as_background_process(
+        return run_as_background_process(
             "delete_old_forward_extrem_cache",
             self.runInteraction,
             "_delete_old_forward_extrem_cache",

synapse/storage/event_push_actions.py

@@ -460,7 +460,7 @@ class EventPushActionsWorkerStore(SQLBaseStore):
             )
 
     def _find_stream_orderings_for_times(self):
-        run_as_background_process(
+        return run_as_background_process(
             "event_push_action_stream_orderings",
             self.runInteraction,
             "_find_stream_orderings_for_times",
@@ -790,7 +790,7 @@ class EventPushActionsStore(EventPushActionsWorkerStore):
         """, (room_id, user_id, stream_ordering))
 
     def _start_rotate_notifs(self):
-        run_as_background_process("rotate_notifs", self._rotate_notifs)
+        return run_as_background_process("rotate_notifs", self._rotate_notifs)
 
     @defer.inlineCallbacks
     def _rotate_notifs(self):

synapse/storage/transactions.py

@@ -273,7 +273,9 @@ class TransactionStore(SQLBaseStore):
         return self.cursor_to_dict(txn)
 
     def _start_cleanup_transactions(self):
-        run_as_background_process("cleanup_transactions", self._cleanup_transactions)
+        return run_as_background_process(
+            "cleanup_transactions", self._cleanup_transactions,
+        )
 
     def _cleanup_transactions(self):
         now = self._clock.time_msec()