[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/paymentserver.cpp

@@ -341,7 +341,7 @@ void PaymentServer::initNetManager()
 
 void PaymentServer::uiReady()
 {
-    assert(netManager != NULL); // Must call initNetManager before uiReady()
+    initNetManager();
 
     saveURIs = false;
     foreach (const QString& s, savedPaymentRequests)