[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
Merge pull request #3187 from Diapolo/netManager

[Qt] paymentserver: start netManager in uiReady()