rtsp-stream: improve handling of sequence numbers

Change so that gst_rtsp_stream_get_current_seqnum() returns the sequence number of the last buffer pulled from the appsink instead of the sequence number in the payloader. If no buffer have been pulled from the appsink (e.g. in the case of UDP) then the sequence number in the payloader is used instead.

This gives a more accurate reading of the current sequence number and ensures that there is no gap in sequence numbers following PLAY-PAUSE-PLAY when using suspend mode GST_RTSP_SUSPEND_MODE_RESET, which would be the case without this change.