Add staging area support to Harbormaster/Drydock + various fixes

Summary:
Ref T9252. This primarily allows Harbormaster to request (and Drydock to fulfill) working copies with a patch from a staging area. Doing this means we can do builds on in-review changes from arc diff.

This is a little cobbled-together but should basically work.

Also fix some other issues:

• Yielded, awakend workers are fine to update but could complain.
• We can't log slot lock failures to resources if we don't end up saving them.
• Killing the transaction would wipe out the log.
• Fix some TODOs, etc.

Test Plan: Ran Harbormaster builds on a local revision.

Reviewers: hach-que, chad

Reviewed By: chad

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14214