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

minor, but let's clean this up:
```scala
dag.nodes.exists {
  case LeftJoinedProducer(l, StoreService(s)) => s == store
  case _ => false
}
```
no need for match on the only input.