Make CachingProvider Closeable

Cache and CacheManager have a `#close` method and extend `Closeable`.
CachingProvider has a `#close` method but does not extend `Closeable`

 * make CachingProvider extend Closeable

Mailing list discussion:
https://groups.google.com/forum/#!topic/jsr107/Z0TbFJm-vkk

I did sign the Terracotta Contributor Argeement.