wishlist: Cache for directories / metadata only / small files only

[chrysn]

For use cases like media (think performous, though I haven't made sure it's really beneficial there) it would be convenient to have the persistent caching store directory listings (maybe including metadata) but not the large files.

(Sure one could go to some lengths of distinguishing cache content that it pays of to cache, but with the filesystem cache also involved and given that it'd be good to know early what to cache and what not, having some heuristics like "it's metadata" or "it's smaller than 4KiB" are probably good enough for many applications).