'Test if `index` is cached.'

def _cached(self, dbname, coll, index):

    cache = self.__index_cache
    now = datetime.datetime.utcnow()
    with self.__index_cache_lock:
        return ((dbname in cache) and (coll in cache[dbname]) and (index in cache[dbname][coll]) and (now < cache[dbname][coll][index]))
