Reintroduce DB changes from #14979, using SCHEMA_COMPAT_VERSION mechanism

[DMRobertson]

The changes in #14979

ALTER TABLE current_state_events ADD COLUMN event_stream_ordering BIGINT;
ALTER TABLE local_current_membership ADD COLUMN event_stream_ordering BIGINT;
ALTER TABLE room_memberships ADD COLUMN event_stream_ordering BIGINT;

INSERT INTO background_updates (update_name, progress_json) VALUES
  ('populate_membership_event_stream_ordering', '{}');

Problems:

• the query used by get_ex_outlier_stream_rows_txn becomes ambiguous: there are two columns called event_stream_ordering. Add event_stream_ordering column to membership state tables #14979 disambiguates this, but won't exist in old code.
• the background update doesn't add an ordering, so it might usurp other updates in the queue.

Questions:

• does the new query assume the populate_membership_event_stream_ordering background update has completed? How long do we expect it will take?

For 1.77 I

• reverted the PR: 9cd7610
• but kept in the change which disambiguates the columns in the source code anyway: f10caa7
• and bump SCHEMA_VERSION: Bump schema version #15036

For the future we should try to land the schema changes gradually (see guidance) over the next few releases.

[DMRobertson]

(cc @Fizzadar)