R1:ee937e99fb9a

Fix unbounded expansion of allocating resource pool

Summary:
Ref T9252. I think there's a more complex version of this problem discussed elsewhere, but here's what we hit today:

- 5 commits land at the same time and trigger 5 builds.
- All of them go to acquire a working copy.
- Working copies have a limit of 1 right now, so 1 of them gets the lease on it.
- The other 4 all trigger allocation of //new// working copies. So now we have: 1 active, leased working copy and 4 pending, leased working…