R1:789df89c84b5

Add a command queue to Drydock to manage lease/resource release

Summary:
Ref T9252. Broadly, Drydock currently races on releasing objects from the "active" state. To reproduce this:

- Scatter some sleep()s pretty much anywhere in the release code.
- Release several times from web UI or CLI in quick succession.

Resources or leases will execute some release code twice or otherwise do inconsistent things.

(I didn't chase down a detailed reproduction scenario for this since inspection of the code makes…