R1:96c51028e50f

In Harbormaster, release artifacts as soon as no waiting/running build steps will use them

Summary:
Ref T11153. If you have a build plan like this:

- Lease machine A.
- Lease machine B.
- Run client-tests on machine A.
- Run server-tests on machine B.

...and we get machine A quickly, then finish the tests, we currently do not release machine A until the whole plan finishes.

In the best case, this wastes resources (something else could be using that machine for a while).

In a worse case, this…