Memcached_DataObject stores empty values in the cache

Memcached_DataObject stores empty values in the cache

There's great value in knowing that something doesn't exist. We
now cache this information, and carefully compare the results from
cache as $results !== false instead of !empty($results), since some
empty values (null, 0, empty array, empty string) are stored in the
cache.

Caching staticGet() and pkeyGet() now store DB misses in the cache,
and cachedQuery() checks for empty results from the cache.