Move index creation to background update (#18439)

Follow on from #18375. This prevents blocking startup on creating the
index, which can take a while

---------

Co-authored-by: Devon Hudson <devon.dmytro@gmail.com>

changelog.d/18439.bugfix

@@ -0,0 +1 @@
+Fix startup being blocked on creating a new index. Introduced in v1.130.0rc1.

synapse/storage/databases/main/sliding_sync.py

@@ -68,6 +68,14 @@ class SlidingSyncStore(SQLBaseStore):
             columns=("membership_event_id",),
         )
 
+        self.db_pool.updates.register_background_index_update(
+            update_name="sliding_sync_membership_snapshots_user_id_stream_ordering",
+            index_name="sliding_sync_membership_snapshots_user_id_stream_ordering",
+            table="sliding_sync_membership_snapshots",
+            columns=("user_id", "event_stream_ordering"),
+            replaces_index="sliding_sync_membership_snapshots_user_id",
+        )
+
     async def get_latest_bump_stamp_for_room(
         self,
         room_id: str,

synapse/storage/schema/main/delta/92/04_ss_membership_snapshot_idx.sql

@@ -12,5 +12,5 @@
 -- <https://www.gnu.org/licenses/agpl-3.0.html>.
 
 -- So we can fetch all rooms for a given user sorted by stream order
-DROP INDEX IF EXISTS sliding_sync_membership_snapshots_user_id;
-CREATE INDEX IF NOT EXISTS sliding_sync_membership_snapshots_user_id ON sliding_sync_membership_snapshots(user_id, event_stream_ordering);
+INSERT INTO background_updates (ordering, update_name, progress_json) VALUES
+  (9204, 'sliding_sync_membership_snapshots_user_id_stream_ordering', '{}');