Improve wire delimiting

[bookmoons]

The current protocol uses the Unicode character ⏎ U+23CE Return Symbol in UTF-8 to terminate messages on the wire. In binary this is the 3 byte sequence e2 8f 8e.

I'm not completely comfortable with this. I don't think there's any guarantee that this byte sequence won't end up in a message.

Update the protocol to use some safe mechanism of message delimiting.

[bookmoons]

The CashShuffle team have agreed on using a length prefix scheme. Specifics to be determined.