R1:718cdc24471a

Implement Build Plan "Hold Drafts" behavior

Summary: Ref T13258. Makes the new "Hold Drafts" behavior actually work.

Test Plan:
- Created a build plan which does "Make HTTP Request" somewhere random and then waits for a message.
- Created a Herald rule which "Always" runs this plan.
- Created revisions, loaded them, then sent their build targets a "fail" message a short time later.
- With "Always": Current behavior. Revision was held as a draft while building, and returned to me for changes…