Add support for optional caching of search query responses

Introduce ?SEARCH_CACHE macro with default value set to
chef_wm_search_cache which can be overrided via compile-time define.

Add no-op chef_wm_search_cache module implementing the search cache
interface but always returning not_found (cache miss).

Modify chef_wm_search to get/put from ?SEARCH_CACHE.
Record cache hit/miss status in request log for search