net: require a verack before responding to anything else

7a8c251901 made this logic hard to follow. After that change, messages would
not be sent to a peer via SendMessages() before the handshake was complete, but
messages could still be sent as a response to an incoming message.

For example, if a peer had not yet sent a verack, we wouldn't notify it about
new blocks, but we would respond to a PING with a PONG.

This change makes the behavior straightforward: until we've received a verack,
never send any message other than version/verack/reject.

The behavior until a VERACK is received has always been undefined, this change
just tightens our policy.

This also makes testing much easier, because we can now connect but not send
version/verack, and anything sent to us is an error.

src/net_processing.cpp

@@ -1420,6 +1420,13 @@ bool static ProcessMessage(CNode* pfrom, const std::string& strCommand, CDataStr
         pfrom->fSuccessfullyConnected = true;
     }
 
+    else if (!pfrom->fSuccessfullyConnected)
+    {
+        // Must have a verack message before anything else
+        LOCK(cs_main);
+        Misbehaving(pfrom->GetId(), 1);
+        return false;
+    }
 
     else if (strCommand == NetMsgType::ADDR)
     {