libceph: support for CEPH_OSD_OP_LIST_WATCHERS

Add support for this Ceph OSD op, needed to support the RBD exclusive
lock feature.

Signed-off-by: Douglas Fuller <dfuller@redhat.com>
[idryomov@gmail.com: refactor, misc fixes throughout]
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Mike Christie <mchristi@redhat.com>
Reviewed-by: Alex Elder <elder@linaro.org>