librbd: add TypeTrait class to librbd/cache

librbd: add TypeTrait class to librbd/cache

- add TypeTraits to librbd/cache
- remove the second template parameter of parent image dispatch
- remove m_cache_client and m_image_ctx to private selection,
  and add two public method to get these two data member.
- modify parent image cache uite testing

Signed-off-by: Dehao Shang <dehao.shang@intel.com>