start_doing_background_updates once and for all databases

The code has evolved since I first put this PR up, see https://github.com/element-hq/synapse/pull/19181

synapse/app/homeserver.py

@@ -449,10 +449,6 @@ async def start(
 
     await _base.start(hs, freeze=freeze)
 
-    # TODO: Feels like this should be moved somewhere else.
-    for db in hs.get_datastores().databases:
-        db.updates.start_doing_background_updates()
-
 
 def start_reactor(
     config: HomeServerConfig,

synapse/server.py

@@ -670,7 +670,8 @@ class HomeServer(metaclass=abc.ABCMeta):
         self.get_task_scheduler()
         self.get_common_usage_metrics_manager().setup()
         start_phone_stats_home(self)
-        self.get_datastores().main.db_pool.updates.start_doing_background_updates()
+        for db in self.get_datastores().databases:
+            db.updates.start_doing_background_updates()
 
     def get_reactor(self) -> ISynapseReactor:
         """