cache results of ResourceResolver.getResourceParentType

In the context of Sling Models I found that a lot of calls to ResourceResolver.getParentResourceType(Resource) are done, which result in repository access. But this can happen also outside of it.

In a simple example (AEM wknd page) I found in total 320 calls to in total 36 resources. Caching these can save around 280 requests to the repository, which a reduction in repository access in the range of 2-3% when rendering the complete page.