createHtlcTx always places the HTLC output at index 0 and the change
output (if any) at index 1. Previously, createHtlcSweepTx attempted to
dynamically find the HTLC index but then unconditionally read
TxOut[0].Value, ignoring the computed index.
Replace the dynamic search with a const htlcInputIndex=0 and a fail-fast
check that errors if the layout invariant is ever violated. Add a test
that verifies the sweep value is derived from the HTLC output, not the
change output.
In this commit we add a new function SelectDeposits
to the loop-in manager. It coin-selects deposits that
meet an arbitrary swap amount provided by the client.
We have to ensure that the server creates the correct
change outputs for the htlc- and sweepless sweep
transactions.
