Reexamine migration table detection/creation

[roji]

In order to fix #2878 for 9.0, #3294 changed our migration history table creation strategy - we try to select from it without pre-checking whether it exists, and catch "table doesn't exist" exceptions, at which point we create it.

This unfortunately doesn't work if the migration process is wrapped in a user transaction - the error causes the transaction to go into a failed state, and everything breaks down. Reexamine our strategy here.

The relevant tasks are MigrationsInfrastructureNpgsqlTest.{Can_apply_two_migrations_in_transaction,Can_apply_two_migrations_in_transaction_async}.

Note: the fix may need to be backported.