Add a new monitor update result handling macro

Add a new monitor update result handling macro

Over the next few commits, this macro will replace the
`handle_monitor_update_res` macro. It takes a different approach -
instead of receiving the message(s) that need to be re-sent after
the monitor update completes and pushing them back into the
channel, we'll not get the messages from the channel at all until
we're ready for them.

This will unify our message sending into only actually fetching +
sending messages in the common monitor-update-completed code,
rather than both there *and* in the functions that call `Channel`
when new messages are originated.