WIP: runtime: Delete old temporary copies in the background
After we have decided to delete an old temporary copy from var/, there is actually no need to continue to hold locks, because the temporary copy is never reused. If we put this in the background, we can continue with other setup in parallel with deleting the temporary copy, saving some time during setup.
/cc @denittis
Pushing this as a possible direction to try resuming at some point, but this doesn't actually speed up setup a whole lot, if at all - presumably the rm -fr
just competes with the rest of what we're doing and makes it slower.