This commit adds a new struct to hold all HTLC keys and refactors the
SwapContract which is used by both loopin and loopout swaps to use this
new struct. The newly added internal keys will for now hold the script
keys to keep everything equivalent but are already stored and read back
if the protocol version is set to MuSig2.
To allow users to specify differing confirmation targets, we store the
swap conf target per-swap. This makes us restart safe, so we do not
forget confirmation values for swaps that are in flight when we restart.
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.
