kfake: add DropControl, SleepOutOfOrder, CoordinatorFor, RehashCoordi…

…nators

* Sleeping was a bit limited because if two requests came in on the same
  connection, you could not really chain logic. Sleeping out of order
allows you to at least run some logic to gate how requests behave with
each other. It's not the most obvious, so it is not the default.

* Adds SleepOutOfOrder

* Adds CoordinatorFor so you can see which "broker" a coordinator
  request will go to

* Adds RehashCoordinators to change where requests are hashed to

The latter two allow you to loop rehashing until a coordinator for your
key changes, if you want to force NotCoordinator requests.