net: advertise support for ADDRv2 via new message

Introduce a new message `sendaddrv2` to signal support for ADDRv2.
Send the new message immediately after sending the `VERACK` message.

Add support for receiving and parsing ADDRv2 messages.

Send ADDRv2 messages (instead of ADDR) to a peer if he has
advertised support for it.

Co-authored-by: Carl Dong <contact@carldong.me>

test/functional/test_runner.py

@@ -154,6 +154,7 @@ BASE_SCRIPTS = [
     'feature_proxy.py',
     'rpc_signrawtransaction.py',
     'wallet_groups.py',
+    'p2p_addrv2_relay.py',
     'p2p_disconnect_ban.py',
     'rpc_decodescript.py',
     'rpc_blockchain.py',