"background" caching

[yarikoptic]

we could run decorated function in the main thread and do "fingerprinting" in a separate thread (or process?), so that if fingerprint found unchanged, we interrupt execution of the main thread and return cached value, if we found that it changed -- we just keep going and store a new fingerprint.
this should be optional since probably would benefit only in the cases where underlying function is not stat-heavy itself. E.g. I would not use this around git status, but it might still be of benefit for datalad status which does git calls, which would operate on index etc, so there would be good amount of time spent on IO/text parsing etc. if we can make both fingerprinting and status run in parallel -- could be faster than fingerprinting + status sequentially.