RFC 629: Harden dbworker store

Created by: efritz

Implements a portion of RFC 629: Cold-storing older worker execution logs.

Ensure that any possible out-of-order operations are caught at the dbworker store layer. Specifically, trying to update execution_log values after the parent record has been "closed" (see this note in the RFC) or stashed should now result in an hard and illustrative error.