osd_types: add osd_perf_stat_t type

Signed-off-by: Samuel Just <sam.just@inktank.com>
ObjectStore: add ObjectStore::get_cur_stats and impl in FileStore

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSDMonitor: add 'osd perf' command to dump recent osd perf information

Signed-off-by: Samuel Just <sam.just@inktank.com>
Merge pull request #500 from ceph/wip-5910

Resolves #5910.

Reviewed-by: Greg Farnum <greg@inktank.com>