Deprecate distinct pseudo-MsgType for first-messages
Deprecate distinct pseudo-MsgType for first-messages

First-packets have a zero-byte for the MsgType: the "real" type of the
first packet is typically a four-byte integer and can be handled
independently.  These constants are just vestiges of an attempt to
pack the few known startup packet types into the core protocol framing
and mechanics.

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

johto/femebe,uhoh-itsmaciek/femebe