Migrations run on every node at boot, and one migration rebuilds an index with
CREATE INDEX CONCURRENTLY, which can't run in a transaction and waits for all
concurrent transactions to drain. The advisory lock that serialises migrations
was held inside an open transaction, so the lock-holder's own idle-in-
transaction connection deadlocked the concurrent build when several nodes
started together. Acquire the lock on an AUTOCOMMIT connection and poll
pg_try_advisory_lock so no waiter pins a snapshot. Adds a Postgres concurrency
regression test (skipped on SQLite).
Patrick Buckley