in the case where an htlc is failed, it could happen
that we use the wrong list of htlcs to generate sweep
tx'es. we would use the pending list instead of the
committed list.
observed by sending 12300sat and then 123000sat,
the second payment fails and an AssertionError was
triggered cause the htlc output could not be found
in the ctx.
added some documentation to clarify the behaviour
of lnchan.included_htlcs.