mon: clear osd request queue latency info on down or up

It doesn't make sense to warn about slow requests on OSDs that are down;
clear those stats in the monitor when we see an OSD go down (or up).

Fixes: #8692
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2055 from ceph/wip-8692

mon: clear osd request queue latency info on down or up

Reviewed-by: Samuel Just <sam.just@inktank.com>