The policy will be asymmetrical for the OSDs, so we need to
track it independently.
The various assertions aren't all worked out yet. Notably,
connection races aren't quite right. But the basic bits
are there.
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.
