Allow settling a channel with any BP that has at least the required nonce

[karlb]

Right now, the BPs submitted during settlement have to match the hashed balance proofs submitted during closeChannel and updateNonClosingBalanceProof. The downside of this is that you need to store all your previous BPs, or you won't be able to settle a channel which has been updated with any BP but the most recent one.

This could be avoided by allowing more recent BPs during the settleChannel call. Only the nonce will be compared, the BP hash does not have to match. The signature has to be valid, of course.