osd: fix object_stat_sum_t dump signedness

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add last_clean_scrub_stamp to pg_stat_t, pg_history_t

Signed-off-by: Sage Weil <sage@inktank.com>
osd: add num_scrub_errors to object_stat_t

Signed-off-by: Sage Weil <sage@inktank.com>
osd: note last_clean_scrub_stamp, last_scrub_errors

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