DRAFT: interactive tx construction protocol

ZmnSCPxj has discussed the possibility of using joinmarket's PoDLEs as a preferable alternative to using a transaction chain with a distinguishable SIGHASH. A question is raised about whether Schnorr signature could be used instead of ECDSA in this protocol. ZmnSCPxj explains that while the signing scheme for PoDLE is not exactly Schnorr, it does use the same schema. The DLEQ proof is based on Fiat-Shamir, which is similar to Schnorr signing, but code reuse is unlikely due to differences in the signing scheme.ZmnSCPxj also considers how PoDLEs could be integrated into niftynei's work on the dual-funded channel proposal, noting that the PoDLE broadcast protocol appears to be the bigger part. However, a wrinkle arises where PoDLE needs to be exchanged if the acceptor wants to add its own funds. If the opener offers to open a channel but the acceptor is not interested in revealing its own funds, then the opener need not reveal PoDLE of its UTXOs. ZmnSCPxj suggests that individual PoDLEs are small enough that a simple gossip protocol might work.