This commit extends SwapResponse and SwapStatus with np2wsh and p2wsh
htlc output addresses to support both nested and native segwit htlcs
in loop-in.
Furthermore the commit adds support for native segwith loop-in htlcs.
When the htlc is paid internally, as of this commit we'll use NP2WSH,
otherwise users are free to select whether to pay the NP2WSH or the
P2WSH htlc.
A database field was already in place to allow channel selection for
loop in. Unfortunately this field, which contains a short channel id,
isn't easily usable for controlling the loop server payment. Because of
non-strict forwarding, it is only possible to constrain a route to a
specific last hop pubkey.
This commit converts the existing field into a pubkey field.
Allow user to specify that htlc will be published by an external source.
In that case no internal htlc tx will be published.
To facilitate sending to the htlc address, the swap initiation response
is extended with that address.
