vod.py: elegant minimal change with which we avoid trashing the

storage when postprocessing recordings. When there are many recordings
taking place at once, easy when using the IPTV_BW feature, at the end,
if there are many processes copying what can be huge files at full
speed all at once, the storage can get a little unresponsive.
Now they will make a queue, and only one postprocessing
mkvmerge/ffmpeg will run at once.

Signed-off-by: Javier Marcet <javier@marcet.info>