OSD: add OSDService::estimate_delay

Signed-off-by: Samuel Just <sam.just@inktank.com>
osd_types: add latency_estimate to osd_stat_t

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: fill in latency_estimate on osd_stat_t

Signed-off-by: Samuel Just <sam.just@inktank.com>
MOSDPing: add latency_estimate

Signed-off-by: Samuel Just <sam.just@inktank.com>
MOSDSubOpReply: add latency estimate

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: maintain set of most recently seen latency for peers

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG: update peer latency from sub ops

Signed-off-by: Samuel Just <sam.just@inktank.com>