Add controls for stopping iperf

[jp-pino]

To prevent downgraded performance on the video stream we stop iperf when we detect connections to the video stream. Currently node_iperf detects this by checking main_camera_rtsp/number_of_connections and /guestport_camera_rtsp/number_of_connections.

When downloading media files through either the Blueye App or the Blunux Web App, node_iperf has no way of knowing not to run. Every minute while downloading media files we'll trigger iperf which will:

• Slow down the downloads (could drop them altogether)
• And skew the data we're trying to get from running iperf in the background, which is to know the max tether speed when the drone is idle.

This PR suggest introducing a SetIperfStatusReq to notify node_iperf that it should stop altogether. When set to true iperf will be executed normally (checking the number of streamers) and when set to false it will override this behavior and stop future iperf checks.