FileStore: add a perf counter for time spent acquiring op queue throttle

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
OSD: add perfcounter tracking messages delayed pending a map

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
ObjectStore: add omap_rmkeyrange to dump

Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>