* Add `event_stream_ordering` column to membership state tables

Specifically this adds the column to `current_state_events`,
`local_current_membership` and `room_memberships`. Each of these tables
is regularly joined with the `events` table to get the stream ordering
and denormalising this into each table will yield significant query
performance improvements once used.

* Make denormalised `event_stream_ordering` columns foreign keys
* Add comment in schema file explaining new denormalised columns
* Add triggers to enforce consistency of `event_stream_ordering` columns
* Re-order purge room tables to account for foreign keys
* Bump schema version to 75

Co-authored-by: David Robertson <david.m.robertson1@gmail.com>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>