Allow developers define whether a page should be cached as a soft reference or not

This ticket is the result of a discussion on the Tapestry mailing list about the consequences of caching page instances using soft references instead of regular ones: https://lists.apache.org/thread/5qbyg0mfscg9qj6jho9899rh4r0kz97z. The goal here is to allow the developer to define which pages should be cached as soft references (the default) or regular ones.

Usage example

public static void contributePageCachingReferenceTypeService(
OrderedConfiguration<PageCachingReferenceTypeService> configuration)

{ configuration.add("VeryLarge", p -> p.equals("VeryLarge") ? ReferenceType.STRONG : null); }

This would cause the page named VeryLarge to be cached using a
regular, strong, non-garbage-collectable reference while leaving all
other pages cached with a soft, garbage-collectable reference.