Since we now use the short_channel_id to identify the next hop we need
to resolve the channel_id to the pubkey of the next hop. This is done
by calling out to `gossipd` and stuffing the necessary information
into `htlc_end` and recovering it from there once we receive a reply.
This lets us link HTLCs from one peer to another; but for the moment it
simply means we can adjust balance when an HTLC is fulfilled.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
