Don't retry joining partial state rooms all at once (#19402)

On restart we retry joining partially stated rooms, but if you have a
bunch in the database this can cause big performance issues if we start
them all at once. So we stagger them.

Author: erikjohnston

changelog.d/19402.misc

@@ -0,0 +1 @@
+Don't retry joining partial state rooms all at once on startup.

synapse/handlers/federation.py

@@ -1788,6 +1788,10 @@ async def _resume_partial_state_room_sync(self) -> None:
                 room_id=room_id,
             )
 
+            # We don't start all the partial state room syncs at once, to avoid
+            # overloading the process.
+            await self.clock.sleep(Duration(milliseconds=10))
+
     def _start_partial_state_room_sync(
         self,
         initial_destination: str | None,