PollingDocumentEventProcessorRoute sends to document endpoint one document at a time; could use batch

[alechenninger]

We made the assumption that the use case is always to produce to this endpoint one at a time, but this is not necessarily true. For instance, you can produce to a JMS endpoint with a batch of messages in one transaction, which would be a more natural fit here since we're already dealing with a message batch.

In any case, this is a lossy translation; if we send as a batch, then the document endpoint can still handle it however it wants, and if it need to split that is still an option. We are not leaving that option on the table with the current design.