NestQuestDatabase rejects a second transaction while one is active, so
two concurrent apply_migrations callers on one connection crashed with
RuntimeError instead of queueing. apply_migrations now holds a
per-(connection, loop) asyncio lock across the whole decide-and-apply
span; the queued runner re-reads the stamped version and applies
nothing. Deterministic concurrent-runner regression test added.
Joshua King
f27c84488d