Support incoming chain swap under/overpayment

[danielgranhao]

An incoming chain swap for which an amount was specified but was over or underpaid, as long as the amount is within the min and max swap limits, can be treated as a zero-amount swap. This means that it doesn't have to be refunded. The payment can be completed if a new quote is fetched and accepted.

[danielgranhao]

In several areas of the codebase, the term "amount less" or "zero amount" swap is used. These can and should be generalized as they apply to the use case mentioned above.

[roeierez]

Discussed that with @kingonly and not sure this should be in priority currently.
The reason is because now that we have amount-less swaps probably it is going to be the main use case and using swaps with amount perhaps will be for cases where the amount is important (e.g user is expecting an exact amount).
@danielgranhao if you agree let's hold it for now.

[danielgranhao]

@roeierez SGTM