R1:78bf266bde4a

Allow Harbormaster build targets to wait for messages

Summary:
This hooks up all the pieces of the build pipeline so `harbormaster.sendmessage` actually works. Particularly:

- Candidate build steps (i.e., those which interact with external systems) can now "Wait for Message". This pauses them indefinitely when they complete, until something calls `harbormaster.sendmessage`.
- After processing a target, we check if we should move it to PASSED or WAITING.
- Before updating a build, we move WAITING…