This change splits out resource transformation out from the
ResourceResolverChain so that chain is focused entirely on resource
resolution (as its name suggests). The invocation of transformers
is left as a separate step, it uses a different (recursive)
algorithm in any case and iterates over a different set of objects.

Also ResourceResolverChain is now limited strictly to methods that
a ResourceResolver should be able to use to delegate to remaining
resolvers.

Furthermore, ResourceResolverChain now maintains an internal index
of the "current" resolver so that resolvers don't have to pass the
chain when invoking it much like a (Servlet API) FilterChain works.
If the last resolver calls the chain again, a null value is returned.