[tests] Move version message sending from NodeConn to NodeConnCB

Summary:
This commit moves the logic that sends a version message
on connection from NodeConn to NodeConnCB. NodeConn should
not be aware of the semantics or meaning of the P2P payloads.

Partial backport of core PR11712 (commit e9dfa9b)

Depends on D2360

Test Plan:

./test/functional/test_runner.py

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2361