Support leftJoin against a store in offline platform

Adds support for performing a leftJoin against a store in the offline platform. Currently supports two cases:
1. sumByKey(store) is not dependent on the leftJoin
2. sumByKey(store) has a limited dependency on the leftJoin - only legal nodes between the join and store are [map | filter | flatMap]Values, i.e. we cannot have any key fanout

Based off of @johnynek's PR: https://github.com/twitter/summingbird/pull/551

I thought we ditched using KeyedProducer in the input position. Can we change this to `Producer[P, (K, V)]` and get everything to compile.