This commit fixes a couple of issues that only occur on the branch we
use for minor releases. The branch doesn't contain all refactors and
cleanups so a couple of very minor things need to be fixed.
Because we don't have all new features of the master branch we also
can't apply the fuzz corpora for the master branch as that can lead to
false positives.
We make sure that HTLCs which have already been decided upon
are resolved before before allowing the external interceptor to
potentially cancel them back. This makes the implementation for
the external HTLC interceptor more streamlined.
This commit introduces a basic integration test for the invoice
HTLC modifier. The test covers scenarios where an invoice is settled with a
payment that is less than the invoice amount, facilitated by the invoice
HTLC modifier.
