R1:3ac99006bfd5

Implement optimistic "slot locks" in Drydock

Summary:
See discussion in D10304. There's a lot of context there, but the general idea is:

- Blueprints should manage locks in a granular way during the actual allocation/acquisition phase.
- Optimistic "slot locks" might a pretty good primitive to make that easy to implement and reason about in most cases.

The way these locks work is that you just pick some name for the lock (like the PHID of a resource) and say that it needs to be acquired for the…