I've been thinking about this one. So, I can understand the desire to move
off of 64-bit UIDs, but I don't understand the immediate jump to 128-bit.
That was my initial thought too when seeing the ticket: why not find
some middle-ground (except that I didn't do the math 
While poking around on this, it seems like current implementation could be
more efficient. Specifically:
(It's not great, but I don't think this should have much of a
performance impact.)