Support leftJoin against a store in offline platform

** work in progress **

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 flatMapValues and merges, i.e. we cannot have any key fanout

Based off of @johnynek's PR: https://github.com/twitter/summingbird/pull/551

TODO: need to resolve the Spark platform matcherror (handling the new ValueFlatMappedProducer)
don't you want to use your sum on line 44 here?