The test framework's p2p implementation currently sends out it's VERSION
message immediately after an inbound connection (i.e. TestNode outbound
connection) is made. This doesn't follow the usual protocol flow where
the initiator sends a version first, and the responders processes that
and only then responds with its own version message. Change that
accordingly by only sending immediate VERSION message for outbound
connections (or after v2 handshake for v2 connections, respectively),
and sending out VERSION messages as response for incoming VERSION
messages (i.e. in the function `on_version`) for inbound connections.
Note that some of the overruled `on_version` methods in functional tests
needed to be changed to send the version explicitly.
Using disconnect_p2ps instead of peer_disconnect makes
the node wait for the disconnect to complete. As a result,
we can reuse p2p_idx=0 in the add_outbound_p2p_connection calls.
