prop289: Add consensus parameters to control new SENDME behavior
This is phase two and phase three of proposal 289. Meaning, this ticket is for implementing those switch.
In phase two, we flip a switch in the consensus, and everybody starts sending payload version 1 sendmes. Payload version 0 sendmes are still accepted.
In phase three, we flip a different switch in the consensus, and everybody starts refusing payload version 0 sendmes.