Fix an issue where newly created Drydock resources could be improperly acquired

Summary:
Ref T9252. This is mostly a fix for an edge case from D14236. Here's the setup:

• There are no resources.
• A request for a new resource arrives.
• We build a new resource.

Now, if we were leasing an existing resource, we'd call canAcquireLeaseOnResource() before acquiring a lease on the new resource.

However, for new resources we don't do that: we just acquire a lease immediately. This is wrong, because we now allow and expect some resources to be unleasable when created.

In a more complex workflow, this can also produce the wrong result and leave the lease acquired sub-optimally (and, today, deadlocked).

Make the "can we acquire?" pathway consistent for new and existing resources, so we always do the same set of checks.

Test Plan:

• Started daemons.
• Deleted all working copy resources.
• Ran two working-copy-using build plans at the same time.
• Before this change, one would often [1] acquire a lease on a pending resource which never allocated, then deadlock.
• After this change, the same thing happens except that the lease remains pending and the work completes.

[1] Although the race this implies is allowed (resource pool limits are soft/advisory, and it is expected that we may occasionally run over them), it's MUCH easier to hit right now than I would expect it to be, so I think there's probably at least one more small bug here somewhere. I'll see if I can root it out after this change.

Reviewers: chad, hach-que

Reviewed By: hach-que

Maniphest Tasks: T9252

Differential Revision: https://secure.phabricator.com/D14272