Fix locking

[Nevon]

await db.select(`select pg_try_advisory_lock(${PG_MIGRATE_LOCK_ID}) as "lockObtained"`)

This does not return just an array with a boolean, but an array containing an object with a lockObtained property. With the existing implementation every caller would think that it acquired the lock and proceed with migrations. Easily reproducible by just programmatically calling the runner multiple times in a Promise.all, and having the migrations add something with a unique constraint (and index name for example):

await Promise.all([runner(...), runner(...),])

This PR also makes sure that the lock is released. When testing this, I found that once the first runner ran, the next ones never managed to acquire the lock on subsequent retries.

[dolezel]

👍 thanks