Improve performance of GPU-less AO-RTSP

[tomskikh]

Remove unnecessary memory copying.

• Create a writable numpy array directly from Gst.Buffer, without copying memory from Gst.Buffer to numpy array and then back to Gst.Buffer.
• Found out if it's possible to pass Gst.Buffer rather than bytes from always_on_rtsp_frame_sink to always_on_rtsp_frame_processor. I.e. check if Gstreamer don't destroy Gst.Buffer when there's existing python reference to it.