Previously we were doing handshake symmetrically: both client and server
were transmitting hello and receiving peer's hello simultaneously.
However this does not allow server to adjust its behaviour depending on
which client (protocol version, protocol encoding, ...) is connecting to it.

-> Rework handshake so that client always sends its hello first, and
only then the server side replies. This matches actual NEO/py behaviour:

https://lab.nexedi.com/nexedi/neoppod/blob/v1.12-67-g261dd4b4/neo/lib/connector.py#L293-294

even though the "NEO protocol" states that

	Handshake transmissions are not ordered with respect to each other and can go in parallel.

	( https://neo.nexedi.com/P-NEO-Protocol.Specification.2019?portal_skin=CI_slideshow#/9/2 )

If I recall correctly that sentence was authored by me in 2018 based on
previous understanding of should-be full symmetry in-between client and
server.

However soon we are going to teach server sides to autodetect client
encoding and adjust server to talk to client via its preferred way.
This needs handshake for client and server to be differentiated.

The protocol needs to be adjusted as well. However I'm not sure it is
going to happen...