R1:d6b937ca2796

Allow external systems to send messages to build targets

Summary:
Ref T1049. Allows external systems to send a message to a build target. The primary intended use case is:

- You make an HTTP request to Jenkins.
- The build goes into a "waiting" state.
- Later, Jenkins calls `harbormaster.sendmessage` to report that the target passed or failed.
- The build continues as appropriate.

This is deceptively complicated because:

- There are a lot of race concerns. We might get a message back from an…