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

We have to pass it explicitly from here: https://github.com/twitter/summingbird/pull/557/files#diff-85d7df9a5f994d907f8d33f2d2635e55R392. Will get rid of the implicit