net_processing: use CNode::DisconnectMsg helper

This is not a pure refactor:
1. It slightly changes the log messages, as reflected in the test changes
2. It adds the IP address to all disconnect logging (when fLogIPs is set)

test/functional/p2p_addrv2_relay.py

@@ -78,7 +78,7 @@ class AddrTest(BitcoinTestFramework):
     def run_test(self):
         self.log.info('Check disconnection when sending sendaddrv2 after verack')
         conn = self.nodes[0].add_p2p_connection(P2PInterface())
-        with self.nodes[0].assert_debug_log(['sendaddrv2 received after verack from peer=0; disconnecting']):
+        with self.nodes[0].assert_debug_log(['sendaddrv2 received after verack, disconnecting peer=0']):
             conn.send_message(msg_sendaddrv2())
             conn.wait_for_disconnect()