osd: 'status' admin socket command

Basic stuff, like what state is the OSD in, and what osdmap epoch are
we on.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1341 from ceph/wip-osd-status

osd: 'status' admin socket command

Reviewed-by: Loic Dachary <loic@dachary.org>