make `InFlightHtlc` take into account blinded data

[jbesraa]

we are changing the behavior to call the scorer for the blinded cases in #2551 and it means scorers need to be aware of None targets. we should probably change InFlightHtlcs to use the candidate id internally and take into account blinded portions such that ScorerAccountingForInFlightHtlcs is aware of in-flight HTLCs on blinded paths.

#2551 (comment)

[TheBlueMatt]

Now that we have a PR for this, its not actually urgent, just something we should do, so removing the milestone.