CURATOR-33 recursive TreeCache recipe

Yeah, I especially wanted feedback on the data access.  I could return a `SortedMap<String, ChildData>` here, that would be more useful right?

In general trying to snapshot any particular point in time is kind of dicey.  I wouldn't want to block updates coming in waiting on a lock; and the only other real option is to make the entire tree COW which seems kind of overkill.

That said I could add an accessor to pull an entire subtree, in simple map from fullPath -> data.