Move pusherpool startup into _base.setup (#7104)

This should be safe to do on all workers/masters because it is guarded by
a config option which will ensure it is only actually done on the worker
assigned as a pusher.

synapse/app/_base.py

@@ -276,6 +276,7 @@ def start(hs, listeners=None):
         # It is now safe to start your Synapse.
         hs.start_listening(listeners)
         hs.get_datastore().db.start_profiling()
+        hs.get_pusherpool().start()
 
         setup_sentry(hs)
         setup_sdnotify(hs)