net: zperf: Fix UDP receiver start/stop operation

This commit fixes restarting of UDP receiver service, along with some
other minor cleanups:

* The core issue was udp_server_running flag not being cleared when
  service was stopped. Fix this by introducing udp_receiver_cleanup()
  which does all of the required cleanups when receiver service is
  stopped. The function is called either when the application stopped
  the service with zperf_udp_download_stop(), or when the service was
  stopped due to error.
* net_socket_service_unregister() was not called on
  zperf_udp_download_stop(), but only from the service callback - that
  would not work in case there's no active communication.
* at the same time, net_socket_service_unregister() would be called from
  the service callback in case of errors. Fix this, by making
  udp_recv_data() only return an error, and let the service callback to
  do the cleanup.
* Remove no longer used udp_server_run semaphore
* Remove udp_server_stop - with socket services it seems no longer
  needed.
* zperf_udp_receiver_init() now returns an error, so that we don't
  mark the service as running in case of socket/services error.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>