R1:619943bea01c

Reduce collision rate for concurrency-limiting slot locks

Summary:
Depends on D19077. Ref T13073. When we're using slot locks to enforce a limit (e.g., maximum of 5 simultaneous things) we currently load locks owned by the blueprint to identify which slots are likely to be free.

However, this isn't right: the blueprint doesn't own these locks. The resources do.

We still get the right behavior eventually, but we incorrectly identify that every slot lock is always free, so as the slots fill up we'll tend…