epg: rework changed channels detection and fixing

 - launch detection after every EPG update
 - fix wrong channel_ids in nfo files in reindex_recordings()
 - rework upgrade_channel_numbers() to also take into account changed
 names and not recognized channel dirs, with auto reindexing
 - simplify does_recording_exist to only cater configured recording
 container (MKV_OUTPUT setting), so recordings done with alternative
 container are not indexed

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