bloom: properly prototype bloom_dump
undup: add count_event and count_stats
bloom: prototype bloom_weight
undup: move count_* to undup-fuse, plumb to UNDUP_STATS, etc
undup: improve count_* interface
 - rebuild .o when .h changes
 - use enum rather than #define
 - add bloom hit/falsepositive counters
 - track how much time we spend doing stats