[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/test/paymentservertests.cpp

@@ -60,7 +60,6 @@ void PaymentServerTests::paymentServerTests()
     X509_STORE_add_cert(caStore, parse_b64der_cert(caCert_BASE64));
     PaymentServer::LoadRootCAs(caStore);
     server->setOptionsModel(&optionsModel);
-    server->initNetManager();
     server->uiReady();
 
     // Now feed PaymentRequests to server, and observe signals it produces: