Check that chapters caching is working when using an external asset

[eteubert]

This looks to me like the transient cache is setup correctly: https://github.com/podlove/podlove-publisher/blob/53b0ee28d0004dfce88c2b15baea3d9114846ef6/lib/chapters_manager.php#L65-L91

But there is a report that sounds like every chapter access results in a http request.