R1:bb28f94f9b64

Reduce garbage-level of Drydock Allocator implementation

Summary:
Ref T9253. The Drydock allocator is very pseudocodey right now. Particularly, it was written before Blueprints were concrete.

Reorganize it to make its responsibilities and error handling behaviors more clear.

In particular, the Allocator does not manage locks. It's primarily trying to reject allocations which can not possibly work. Blueprints are responsible for locks. See some discussion in D10304.

NOTE: This code probably doesn't work…