"background" caching

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.