CURATOR-33 recursive TreeCache recipe

I haven't looked into zxid order at all; I wish that all event callbacks including a zxid but they don't.  I had consider that perhaps the UPDATE case should compare the mzxid of the incoming event to the mzxid recorded on the existing node, but in practice I don't think events can come in such an out of order fashion.

Given that PathChildren doesn't utilize mzxid's at all, it seems like it'd be more of an assertion than actually useful.