DRAFT: interactive tx construction protocol

The email discusses improving protocol messages by adding a serial ID to inputs and outputs, which can be used for deletions and as an ordering heuristic for transaction inputs during construction. The initiator's serial IDs must be odd, while the non-initiator's serial IDs must be even. Serial IDs should be chosen at random, and collisions between peers are considered protocol errors. Additionally, all output scripts must be standard, and nLocktime is always set to 0x00000000. If a blockheight is communicated, it is set to blockheight-6; otherwise, it is set to zero. The purpose of subtracting 6 is unclear. In multiparty constructions, the initiator must flip the bottom bit of any received inputs before relaying them to a peer. Finally, RBF eligibility will be flagged for all transactions, but RBF and privacy do not work well together.