[Qt] paymentserver: start netManager in uiReady()

- remove explicit init of netManager as this is done in the constructor
  anyway
- move initNetManager() call to uiReady(), which removes an assert() and
  allows us to use message() in initNetManager() (currently unused but
  could be necessary because of proxy related messages)
- make initNetManager() private
- update paymentservertests.cpp

src/qt/bitcoin.cpp

@@ -308,7 +308,6 @@ int main(int argc, char *argv[])
 
                 PaymentServer::LoadRootCAs();
                 paymentServer->setOptionsModel(&optionsModel);
-                paymentServer->initNetManager();
 
                 if (splashref)
                     splash.finish(&window);