Request ID should be part of header

[firepear]

There should be a request id (or sequence number) as part of the header. It should be generated by the client, rather than tracked as an incrementing counter by the server.

[ LENGTH (32bit) | SEQ (32bit) | HMAC (32 bit, optional) | PAYLOAD ]

[firepear]

This is done, except that SEQ comes before LENGTH (such that it does not affect length).

I've just now realized that HMAC, which is also fixed-length, can be moved to precede LENGTH as well.

SEQ (4B) | HMAC (32B, opt) | LENGTH (4B) | PAYLOAD

[firepear]

And now THAT is done.

But I want to clean up variable names and comments, so that everything has clear and consistent names. Currently there's too much swapping between "message", "request", "response", and variants when referring to the payload.

A request or response is something that exists from the POV of the client or server. From inside the code talking to a socket (petrel-net.go), it's a transmission. And transmissions have these parts:

• SEQ
• HMAC
• PLEN (payload length)
• PAYLOAD

[firepear]

• [x] The smart thing to do would be to also include a one-byte version indicator.
• [x] Also, instead of appending thing after thing after thing to the payload, keeping multiple copies of it around in various states, the headers should all be stitched together, then prepended just before transmission
• [x] On the receiving side, the entire header should be removed in one go, then sliced up as needed. We'll always know the length of the header because we know if there's an HMAC or not.