WIP: pv-runtime: Reference-count lock data structures

Simon McVittie requested to merge wip/smcv/refcount-locks into main

This lets more than one thread hold a reference to the lock, with the lock only being released on last-unref.

We don't need this yet - I implemented it when I was thinking about maybe doing runtime cleanup in a background thread, which would benefit from giving that thread a reference to the lock.

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