Adjust the Drydock allocator to limit each pending lease to one allocating resource

Summary:
Ref T13677. Currently, one lease may cause multiple resources to allocate simultaneously if it starts allocating one, then wakes up from a yield later on and still sees no available resources.

This is never desired -- or, at least, produces desirable behavior only entirely by accident. Normally, it causes an excess of resources to allocate.

This is not a catastrophic problem: the extra resources usually get used sooner or later or cleaned up; and the total amount of badness is limited by overall resource allocation limits.

However, this behavior is also suppressed by an artificial "25% of current pool size" growth limit throttle which I intend to remove. Removing this throttle without fixing the allocator behavior could make this "too many resources" problem worse.

Change the allocator so that a lease that has started allocating a resource won't allocate another resource until the first resource leaves the "pending" state.

This also fixes some general oddness with the allocator and attempts to simplify the structure.

Test Plan:

• Ran 8 taskmasters.
• Destroyed all resources and leases.
• Leased 4 working copies.
  • Saw exactly 4 resources build and lease, all simultaneously.
• Destroyed all resources and leases.
• Leased 32 working copies.
  • Saw exactly 32 resources build and lease, approximately 8 at a time (limited by taskmasters).
• Destroyed all leases (but not resources).
• Leased 32 working copies, saw them satisfied by existing resources.

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Maniphest Tasks: T13677

Differential Revision: https://secure.phabricator.com/D21807