Support for creating a store-service for Storm platform

To be able to do leftJoin on a store we need the same object to serve as a store and a service. This is already supported in the Scalding Platform (https://github.com/twitter/summingbird/blob/1b630690a8efe554ab3fabc9b60965c2c95e2e83/summingbird-scalding/src/main/scala/com/twitter/summingbird/scalding/Service.scala#L46). This adds a trait to the Storm Platform that is of both P#Store and P#Service types. This can be used in jobs that use leftJoin against store. Users in their jobs can call:

```
Storm.storeService(mergeableStore, serviceStore)(batcher)
```
where the mergeableStore is the store for aggregation (delta store) and serviceStore is the store that we use as a service (can be delta store or the total store). For read-modify-write behavior the serviceStore should include the delta store, i.e. serviceStore can be a client store that wraps the delta mergeableStore or a client store that wraps the delta mergeableStore and the offline store (i.e. the total store). 
I have serviceStore function in Storm platform that takes the lazy parameters and turns them into functions https://github.com/twitter/summingbird/pull/563/files#diff-36ff64139544fb94baad353028959a1eR83. I was assuming users to be using that function like Storm.store and Storm.service.

For lookups, wouldn't we always want the latest value for a specific key? I agree that it would be nice to pass just one MergeableStore to remove the possibility of getting two different stores. ClientMergeableStore would work in this case.