[BIP] Normalized transaction IDs

While some believe that specifying flags upfront in the UTXO-creating transaction could allow for implementation of fully malleability-proof wallets, others argue that the use of non-sighash_all flags still poses a threat to the security of transactions. Signer malleability is also a concern needing consideration, and it is suggested that wallets should actively CoinJoin, bump fees on, etc any pending transactions in the background to prevent these forms of malleability. The discussion shows that malleability remains an important issue in Bitcoin transactions that needs continued attention.