Throttle requests on server rather than requiring client to do flow control

[hammer]

It would be nice if hackery (I mean that in the kindest way @tavinathanson) like https://github.com/hammerlab/discohorts/commit/ba08d1db9d7b9e4ce14e4ca0cda319fac588c85b was not necessary

[tavinathanson]

👍 Would love it if discohorts didn't need to know/worry about this.