Transaction isolation on DB stores is too strict

The transaction isolation used in the scheduler's DB-based storage is too strict. This results in inconsistent data when work in a transaction forks asynchronous work on a different thread, which is implicitly done for messages on the event bus. In cases like TaskThrottler which has some preconditions around expected state, we encounter exceptions related to the ancestor state not yet being shown as terminated as we try to calculate the reschedule delay.