msgr: include priority in msg header, make dispatch a priority queue

msgr: include priority in msg header, make dispatch a priority queue

Generalizes previous hack that put messages from the monitor at the front of the
dispatch queue.  Monitor now just sets a (non-default) default_send_priority
of CEPH_MSG_PRIO_HIGH.  That value is used only if the message priority isn't
set explicitly by set_priority() before being queued for send.