We used to not print what happened with an HTLC in the `pay`
plugin. This meant that to follow the HTLCs we'd have to map the `pay`
HTLCs to the `lightningd` HTLCs, and then trace that. BY having `pay`
print the outcome as it sees it, we can make that tracking much
simpler, even allowing for tooling to do it for us.
Changelog-None This is a log-only change
Any file in this directory which is executable and whose name only
consists of alphanumeric characters, space, '.', '-' or '_' will be
automatically loaded when lightningd starts (unless suppressed with
commandline options).