osd: use unlock_guard for unlock osd temporarily

osd: use unlock_guard for unlock osd temporarily

when OSD::do_command() gets called, osd_lock is acquired. but when
serving some of these commands, we need to call methods which also
acquire the osd_lock by themselves. for instance,
OSD::handle_conf_change() gets called by cct->_conf.apply_changes().
to allow them to do so, we unlock osd_lock before calling those methods,
and re-lock it after done with them.

unlock_guard is introduced to unlock and re-lock the lock in a RAII style.

Signed-off-by: Kefu Chai <kchai@redhat.com>