Introduce MultisigKeyBag to expose P2SH redeem data

For married wallets KeyChainGroup now keeps redeem scripts together with keys and exposes this data via MultisigKeyBag interface. In future this interface may be used by signers to get data required for signing P2SH transactions.

Ah, I see. Right, I forgot the details around why it works that way, I think something may depend on it. Could you just add a // TODO comment here stating that use of exceptions should be removed?