Implement database locking for migrations

[AndriySvyryd]

Generally speaking, concurrent invocations of Migrate() will fail and possibly leave the database in a corrupted state. In order to mitigate this, we could create a temporary table using HistoryRepository that will serve as a lock while the migrations are applied by the process that acquired it. Providers will be able to override all related logic to instead leverage database-specific mechanisms to accomplish this.

[roji]

That's a good idea - databases do frequently expose locking mechanisms that could work well, e.g. full-table exclusive lock on the HistoryRepository table for PG and SQL Server. Though I'm not sure what a default implementation would look like here - what do you have in mind with the temporary table?

[AndriySvyryd]

what do you have in mind with the temporary table?

Before applying migrations create __EFLock table. If it was created successfully - the lock was acquired. If not, enter the normal retry loop.
After migrations and seeding (#17568) finished applying delete __EFLock table.