[PSBT] TX modifying methods don't throw when finalized

[junderw]

Currently, PSBT checks if setVersion, setLocktime, setInputSequence, addInput, addOutput can be performed by checking the existence of partialSigs, and what sighash they use.

However, after running finalizeAllInputs but BEFORE running extractTransaction, you can change the transaction, and it will extract an invalid transaction (since the transaction changed since the signature)...

Some ways to fix this:

1. fail partialSig check if final* attributes are present.
2. if no partialSig but final* exists, parse final* and search for any signatures and check the sighash (simple fix, but very performance intensive (it checks all inputs every time))

1 seems simple, but there might be a use case where someone wants to pass around PSBTs with only SOME finalized inputs that have SIGHASH_ANYONECANPAY etc. and want to allow other people to add more inputs etc.

In which case 2 is more correct.