Add LRU-based cache size limiting

[tvierling]

This would make it possible to limit the amount of cache space used for local files.

The algo should be least-recently-used, meaning that the atime would be needed in order to determine last access. This should still be possible with a fs mounted "relatime" (though not "noatime") on Linux; and memory-based filesystems (e.g., tmpfs) usually have atime support always enabled. (Make sure to document this requirement as part of the configuration.)

[tvierling]

Seems some newer kernels default to relatime, so this may require setting a "strictatime" mount option to force atime updates.