[entity/optimization] Cache rset when entity.related is called with entities=False If fail to see why we couldn't cache in this case, while this is important for optimization reason: when doing a single HTTP request, some queries are done two or three times because of predicates / uicfg or alike. Also always store tuple and not list in the cache, because: * else we get some regression * and inconsistent result type (tuple or list) * and it simply feels better to cache an unmutable object.

cubicweb source: source-is-missing cubicweb/web/data/jquery.flot.js line length is * characters (>512)