DAP-16: Rework Helper States

[jcjones]

While in #4305 we're going to remove AwaitingRequest, until then we should make the Helper follow the state updates per DAP-16, so we can unblock #4320.

This will involve ensuring the Helper doesn't rely on the writer's safety net to move into a terminal state, and allow for re-acquisition of AWAITING_REQUEST state-jobs in the DB.