This commit adds a new bucket to save liquidity parameters. We've
skipped the serialization and deserialization implementations here and
leave them to be handled by the liquidity package.
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.
