WIP caching empty search. TODO: invalidate cache.
Simpler coding.
Rename variale.
Set caching when needed, and invalidate when time passes.
check_plain().
Add 'facetapi_vocabulary_cache_timeout' drupal variable with 300 (5 minutes) as default.
Merge pull request #5728 from openscholar/5716

Caching "Taxonomy by filter" empty search