Channel directions can be individually active or inactive, which affects
routing directions.
Thus, a dowsing request in on direction may have a different result in
the opposite direction.
Existing clients of the `Dowser` assume the first argument is the source
(proposal) and the second argument is the destination (patron), but the
`Dowser` used the first argument as the destination instead.
This also flips the `Dowser` direction.
