Implement all stubbed functions.
Add an instance counter that pings running secondary instances to ensure they are alive.
Run the entire server response logic in a thread, so the SingleApplication primary server is responsive independently of how busy the main thread of the app is.
Tests?

REMOVE:
  SingleApplicationPrivate::randomSleep();
  quint16 SingleApplicationPrivate::blockChecksum()

  Remove Mode::SecondaryNotification flag. A notification is always sent.