R1:f0fdcf1a516c

Undumb the Drydock resource allocator pipeline

Summary:
This was the major goal of D3859/D3855, and to a lesser degree D3854/D3852.

As Drydock is allocating a resource, it may need to allocate other resources first. For example, if it's allocating a working copy, it may need to allocate a host first.

Currently, we have the process basically queue up the allocation (insert a task into the queue) and sleep() until it finishes. This is problematic for a bunch of reasons, but the major one is that if…