WICKET-5527 Inefficient DefaultPageStore.SerializedPagesCache

Introduce PerSessionPageStore.
Extract the common logic between DefaultPageStore and PerSessionPageStore into AbstractPageStore.
Extract interface for SecondLevelPageCache implementations
WICKET-5527 Inefficient DefaultPageStore.SerializedPagesCache

Extract AbstractCachingPageStore - an AbstractPageStore that uses SecondLevelPageCache to manage the caching
WICKET-5527 Inefficient DefaultPageStore.SerializedPagesCache

Clean up and javadoc