Kismet: a Second Chance cache for files on disk

Kismet implements multiprocess lock-free crash-safe and (roughly) bounded persistent caches stored in filesystem directories, with a Second Chance eviction strategy. The maintenance logic is batched and invoked at periodic jittered intervals to make sure accesses amortise to a constant number of filesystem system calls and logarithmic (in the number of cached file) time complexity, without requiring any persistent in-memory statistics. That's good for application simplicity and performance, and enables lock-freedom, but does mean that caches are expected to temporarily grow past their capacity limits, although rarely by more than a factor of 2 or 3.