Fix race in peer connection in C++ demo app

When we disconnect then immediately reconnect from one peer we may
still get the second connection through before the disconnection is
handled on the other end, causing connection failures. We fix this
here by disconnecting on both ends before reconnecting.

Author: TheBlueMatt

lightning-c-bindings/demo.cpp

@@ -242,6 +242,7 @@ class PeersConnection {
 
 		// Then disconnect the "main" connection, while another connection is being made.
 		PeerManager_disconnect_by_node_id(&net1, ChannelManager_get_our_node_id(&cm2));
+		PeerManager_disconnect_by_node_id(&net2, ChannelManager_get_our_node_id(&cm1));
 		assert(!socket_connect(node1_handler, ChannelManager_get_our_node_id(&cm2), (sockaddr*)&listen_addr, sizeof(listen_addr)));
 
 		std::cout << __FILE__ << ":" << __LINE__ << " - " << "Awaiting new connection handshake..." << std::endl;