Handle the prior knowledge preface
The preface is the magic string "PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n"
which is intended to cause HTTP/1.x connections to fail gracefully.
For clients, this involves sending the magic string and for servers
this consists of receiving the magic string. The return value of this
function is any unconsumed inbound data.
Perform the HTTP/2 prior knowledge handshake
Perform pipeline manipulations necessary upon successful handshaking