CC-4989: upgrade to mutagen 1.21

-done
CC-4915: Media-Monitor cannot handle rabbitmq restart event

-fixed
CC-4916: Pypo cannot handle Schedule updates after rabbitmq restart happens

-fixed
CC-4915: Media-Monitor cannot handle rabbitmq restart event

-fixed
CC-4758: File removed before Liquidsoap finishes reading it?

-fixed
Merge branch '2.3.x' into cc-4979-add-fourth-switch