Handle Runs with no Stop Documents.

[gwbischof]

MongoConsumer get documents from a kafka partition and then inserts them into the correct mongo database with suitcase-mongo.Serializer.

Each time a stop document is received the Serializer is closed, and a new Serializer is created for the next set of documents it will receive. If a run does not have a stop document, then the following run will end up using the previous Serializer. So, as is, this Serializer may serialize more than one run. For the suitcase-mongo.Serializer, this shouldn't matter. But for a suitcase for an archival format, we only want one run per file, and this will be a problem.

So I think we should add some code, hopefully to the base class that handles runs with missing stop documents. This could work by checking if a second start document is received before receiving the stop document.