Improve Drydock's ability to allocate leases correctly

Summary:
Right now, Drydock gives out multiple leases to the same working copy and gives out leases to working copies with repository "P" in them when the user requested some other repository.

Add two callbacks:

• canAllocateLease() - allows a blueprint to reject a lease on a resource because of a fundamental incompatibility, like "it's a working copy with Phabricator in it, but the lease wants a working copy with Javelin in it".
• shouldAllocateLease() - allows a blueprint to reject a lease on a resource because of resource limits, like "only one active lease can own a working copy at a time".

Also cleaned up various other things.

Test Plan:
After implementing the callbacks, Drydock has the correct behavior:

• It gives multiple leases on localhost, but only one lease per working-copy resource.
• It does not grant leases on resources with repository X to requests for repository Y.

Ran bin/drydock lease --type working-copy --repositoryID 12 and similar repeatedly and verified results in the web console.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D4166