net: fix race condition in self-connect detection

Initiating an outbound network connection currently involves the
following steps after the socket connection is established (see
 `CConnman::OpenNetworkConnection` method):
    1. set up node state
    2. queue VERSION message
    3. add new node to vector `m_nodes`

If we connect to ourself, it can happen that the sent VERSION message
(step 2) is received and processed locally *before* the node object
is added to the connection manager's `m_nodes` vector (step 3). In this
case, the self-connect remains undiscovered, as the detection doesn't
find the outbound peer in `m_nodes` yet (see `CConnman::CheckIncomingNonce`).

Fix this by swapping the order of 2. and 3., by taking the `PushNodeVersion`
call out of `InitializeNode` and doing that in the `SendMessages` method
instead, which is only called for `CNode` instances in `m_nodes`.

Thanks go to vasild, mzumsande, dergoegge and sipa for suggestions on
how to fix this.

src/test/util/net.cpp

@@ -28,7 +28,8 @@ void ConnmanTestMsg::Handshake(CNode& node,
     auto& connman{*this};
 
     peerman.InitializeNode(node, local_services);
-    FlushSendBuffer(node); // Drop the version message added by InitializeNode.
+    peerman.SendMessages(&node);
+    FlushSendBuffer(node); // Drop the version message added by SendMessages.
 
     CSerializedNetMsg msg_version{
         NetMsg::Make(NetMsgType::VERSION,