R1:7168d8edd971

Make Drydock reclaim unused resources when it reaches a resource limit

Summary:
Fixes T9994. Currently, when Drydock can't allocate a new resource because some limit has been reached, it waits patiently for a resource to become available.

It is possible that no resource will ever become available. Particularly with "Working Copy" resources, the new lease may want a copy of `rB`, but the resource may already be maxed out on `rA`.

Right now, no process exists to automatically reclaim the unused `rA`.