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

Talked offline, needed to fail when we saw an invalid dag. This is the wrong place to put it, will put it in the case match on the Producer nodes.