R1:449617692489

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…