fix race on disconnect

between the time we check for `is_open` and the actual disconnect, the
socket could already have been disconnected by another thread (TOCTOU).
Furthermore, the disconnects can race causing a segfault or similar
issue in the asio's internals.

src/TServer.cpp

@@ -224,7 +224,7 @@ void TServer::GlobalParser(const std::weak_ptr<TClient>& Client, std::vector<uin
     case 'p':
         if (!Network.Respond(*LockedClient, StringToVector("p"), false)) {
             // failed to send
-            LockedClient->Disconnect("Failed to send ping");
+            Network.DisconnectClient(*LockedClient, "Failed to send ping");
         } else {
             Network.UpdatePlayer(*LockedClient);
         }