WIP: Consumer now multiplexes topic-partitions [breaks API]

Certain services of ours are now using over 1000 connections, and things are getting confused, so we need to multiplex topic-partitions on a FetchRequest. This is an implementation of that.

Mostly a resurrection of my first crack at it.

I'm going to review the configuration parameters; I'd like the defaults to be more sensible.

@Shopify/stack