getNodeTypes should not go to backend on repeated calls

[dbu]

each call to get a specific node type does a request to jackrabbit. first calling $session->getWorkspace()->getNodeTypeManager()->getAllNodeTypes(); reduces that to a single call which is about the same speed (not very fast though, at least with magnolia).

in jackalope-doctrine-dbal, we have the CachedClient that uses a Doctrine\Common\Cache to cache meta data and other things. i think our best approach would be to convert this CachedClient to a pure decorator pattern, instead of extending a Client, and move it to jackalope/jackalope. then it can be used with jackalope-jackrabbit as well.

(moved here from jackalope/jackalope#312)