Add table to track migration of secondary indexes on the event_log table
For SQL-event log implementations, adds a check for event_log migration status.
The instance implementations fronts those migration status queries with an in-memory cache.
This does mean that dagit would need to be restarted to pick up any migration change, but
seems worth it.
Name change suggestions welcome.
Test Plan: BK
Reviewers: alangenfeld, max
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D4800