Consider currently confirmed FundingScope when claiming commitments

Once a commitment transaction confirms, we may have outputs to claim. To
determine whom the commitment transaction belongs to, we generally
compare its `txid` against what we know be ours and the counterparty's.
This, however, relies on being able to match on the expected
`FundingScope`, such that we can produce the necessary output claims.
Since a commitment transaction confirming implies that the funding
transaction it spends has also confirmed, we rely on
`alternative_funding_confirmed` to obtain the corresponding
`FundingScope`.

Author: wpaulino