osd: include op queue age histogram in osd_stat_t

This includes a simple power-of-2 histogram of op ages in the op queue
inside osd_stat_t.  This can be used for a coarse view of overall cluster
performance (it will get summed by the mon), to identify specific outlier
osds who have a higher latency than the others, or to identify stuck ops.

Signed-off-by: Sage Weil <sage@inktank.com>