messages: add MOSDMarkMeDown

Signed-off-by: Samuel Just <sam.just@inktank.com>
ceph_features: add feature bit for osd shutdown reporting

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSDMonitor: factor out check_source helper

Signed-off-by: Samuel Just <sam.just@inktank.com>
Monitor: add MOSDMarkMeDown support

Signed-off-by: Samuel Just <sam.just@inktank.com>
OSD: notify mon prior to shutdown

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