msg: add priority to dispatcher invocation order

msg: add priority to dispatcher invocation order

So we can ensure that e.g. MDSRank::ms_dispatch is lowest priority so that we
do not acquire the mds_lock when looking at beacons.

This change maintains the current behavior when the priority is unset: the use
of std::stable_sort will ensure that the add_dispatcher_head and
add_dispatcher_tail calls will preserve order when dispatcher priorities are
equal.

Fixes: 7fc04be9332704946ba6f0e95cfcd1afc34fc0fe
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>