Add a dated fee budget to our swap suggestions. This budget only applies
to automatically dispatched swaps (which will be added in later
commits). We choose to apply the fee budget to our suggestions so that
they perfectly replicate what the autolooper would do. The budget has a
start date so that it can be refreshed once it has been used up over a
period (rather than having to endlessly increase it).
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.
