ZF-5514: Function and Class frontends always call test() before load()
With some backends (like File) this is efficient., as test() is more efficient than load(). On other backends (like memcached), test() and load() are essentially the same (call _memcache->get), which means that for every cache hit there are two roundtrips to memcached. With such backends, the frontend should just call load() and see if it returns anything.
getCapabilities() can be potentially used to communicate backend's preference to the frontend