Use thimble

WIP, with some review points so it doesn't get too off-track.
Yeah. Since the old interface didn't take an explicit pool (and instead just messed with the reactor pool), if we use the reactor pool too, we can't see if the txkazoo issues we're seeing right now are somehow related to using the reactor pool. (I don't think that's a credible cause, but it's one of the few this experiment may tell us about.)