in _missing(), avoid use of the slow update_stats_from_disc() unless necessary, and avoid creating too many log_events if we have thousands of output files in parse()
in _missing(), avoid use of the slow update_stats_from_disc() unless necessary, and avoid creating too many log_events if we have thousands of output files in parse()

wtsi-hgi/vr-pipe,wtsi-hgi/vr-pipe