This deduplicates code for sending out the VERSION message
(if available and not sent yet), currently used at three
different places:
1) in the `connection_made` asyncio callback
(for v1 connections that are not v2 reconnects)
2) at the end of `v2_handshake`, if the v2 handshake succeeded
3) in the `on_version` callback, if a reconnection with v1 happens