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.
This commit adds the required code to persist loop in swaps. It also
introduces the file loop.go to which shared code is moved.
Sharing of contract serialization/deserialization code has been
reverted. The prepay fields do not apply to loop in, but were part of
the shared contract struct. Without also adding a migration, it wouldn't
be possible to keep the shared code.
In general it is probably more flexible to keep the contract
serialization code separated between in and out swaps.
