doc/dev/msgr2: include stream ids

doc/dev/msgr2: include stream ids

- Add stream ids to the protocol.  This will allow us to
  multiplex multiple entities over the same TCP
  connection.

- Add a frame length.  This lets us pull entire frames
  off the wire without any introspection.  This is
  helpful on the implementation side because each stream
  may have a different cipher and for performance reasons
  because we want to pull large chunks from the network
  at a time.

- Simplify to a single frame format description, with
  only one optional variation (signature or no
  signature).

Signed-off-by: Sage Weil <sage@redhat.com>