Update spec to reflect record-layer-like header

[zanebeckwith]

We want to use a constant header format for all messages (both handshake and session), to both ease parsing and allow for long-message framing.

This means the header should include the total-packet-length in a static location (to ease parsing), even for handshake messages whose structure and length are known.

It should also include framing info (e.g. frame number, total frame count). Cf. the (D)TLS specs for inspiration.

[zanebeckwith]

The spec now does use a common header format, that has the total length in a static location, for all message types.

However, the framing information question has not been resolved. That should probably be a session-only field(s).