This commit detaches signaling the invoice registry that an htlc was
locked in from the actually settling of the htlc.
It is a preparation for hodl invoices.
Previously it was difficult to use the invoice registry in unit tests,
because it used zpay32 to decode the invoice. For that to succeed, a
valid signature is required on the payment request.
This commit injects the decode dependency on a different level so that
it is easier to mock.
This commit is a step to split the lnwallet package. It puts the Input
interface and implementations in a separate package along with all their
dependencies from lnwallet.
