Feed caching fix

For multiuser scenarios (considering that every user may have different feed manipulation plugins active) the currently implemented caching logic will favor the first user and even re-apply the feed fetched hooks.

For the same reasons I deactivated passing the FeedParser object as it will simply ignore the activated fetch feed and feed fetched hooks for any user other than the first one encountered. You could add a check to only pass the object if the next user has the same plugins activated.

Imho considering the likelihood of two users having the same feed manipulation plugins activated its probably not worth the effort.