Incremental LTO will usea cache to store object files.
This patch handles the pruning part of the cache, exposing
a few knobs:
- Pruning interval: the implementation keeps a "timestamp" file in the directory and will scan it only after a given interval since the last modification of the timestamp file. This is for performance purpose, we don't want to scan continuously the folder.
- Entry expiration: this is the time after which a file that hasn't been used is remove from the cache.
- Maximum size: expressed in percentage of the available disk space, it helps to avoid that we blow up the disk space.