reduce queries for fetching many node`s data

As described in the cmf-mailing list (https://groups.google.com/forum/#!topic/symfony-cmf-devs/Bv3REKYwT80), this would be my first changes for reducing queries. There is still an test issue i will solve later.
Is the purpose in here to get all children of `/` and is that really needed or just an test case? All other tests are fine and working.