osd/osd_types: add recovery rates to object_stat_sum_t

Make reates in per-minute units to improve precision.

Signed-off-by: Sage Weil <sage@inktank.com>
common/RateMeasure: class for measuring a rate over time

Keep a short history.  Quantize samples.  Extrapolate when there is limited
historical data.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: measure recovery rates, include in pg_stat_t

Track objects, bytes, and keys.

Signed-off-by: Sage Weil <sage@inktank.com>
mon/PGMap: report recovery rate

Signed-off-by: Sage Weil <sage@inktank.com>