Add support for cache cleanup

[cipriancraciun]

At the moment the z-run cache (used to hold compiled libraries) is not cleaned automatically and it tends to build-up.

Add support for either automatic or on-demand cleanup of cache files.

Care must be taken so that currently active z-run instances aren't left without their in-use library. Thus in addition to the cleanup, we should implement a trivial mark-hot (for example at startup if older than a minute) and then garbage-collect the remaining files.