ceph: use connection mutex to protect read and write stages

ceph: use connection mutex to protect read and write stages

Use a single mutex (previously out_mutex) to protect both read and write
activity from concurrent ceph_con_* calls.  Drop the mutex when doing
callbacks to avoid nested locking (the callback may need to call something
like ceph_con_close).

Signed-off-by: Sage Weil <sage@newdream.net>