Only update response cache when needed

[Will Lunniss]

Store a cache id and check against the server to see if anything has changed.

This requires a change to Serviio to provide per meda type ids to track changes https://bitbucket.org/xnejp03/serviio/issue/714/cds-api-per-media-type-update-ids-to-allow

[Will Lunniss]

Now uses per media type eTags in Serviio 1.4+ to only update the cache when needed