Internode messaging handshake sends wrong messaging version number

With the refactor of internode messaging to netty in 4.0, we abstracted the protocol handshakes messages into a class and handlers. There is a bug when the initiator of the connection sends, in the third message of the handshake, it's own default protocol version number (MessagingService.current_version), rather than the negotiated version. This was not causing any obvious problems when CASSANDRA-8457 was initially committed, but the bug is exposed after CASSANDRA-7544. The problem is during rolling upgrades of 3.0/3.X to 4.0, nodes cannot correctly connect.