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.
i think returning an empty array makes no sense here. $row contains the node we read from the database. this method fetches the list of children so that we know them as well.

looking at the current master code, this seems to be quite a mess when we do pre-fetch, as we query twice for the same thing. i guess the reason to not always get the children in the same query is to avoid loading potentially large child nodes when we only care about the name.

to debug the test failure, i propose trying to see what happens if you put this method back to the previous behaviour (having the duplication). if it still fails, its a problem in getNodesData, otherwise its the above code.