Improve internal XSL cache

Idea created by stephan.burkard on Jun 6, 2016

    Note: Content reworked since it was also referenced for another shortcoming of the XSL cache


    The internal XSL cache of Layer7 is simply not manageable.

    1. It is not possible to clear the cache as an atomic operation (only ugly and error-prone workarounds are possible).
    2. It caches whatever is the result. If I monitor a URL to get the XSL and the request to retrieve the XSL has a timeout, this is effectively cached. Every further call complains about the timeout without trying to access the XSL.
    3. All the nice features known from the HTTP routing assertion are not available when I get an XSL through a URL.