Otherwise what the hook sees is actually a lie, and if it sets it
we might override it.
The side effect is that we add an explicit "forward_to" field, and
allow hooks to override it. This lets a *hook* control channel
choice explicitly.
Changelod-Added: Plugins: `htlc_accepted_hook` return can specify what channel to forward htlc to.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
