Fetch related improvements

[thomasst]

• Allow passing extra filters to be able to specify shard keys per document class.
• Allow specifying custom batch size which also limits the __in query. I ended up using the same value for all document types for simplicity, since I don't really see a use case where we'd want to have different batch sizes per document type. But I can change it if you think it's needed.

[thomasst]

Let's get this merged. I propose we just merge this as is, but if you feel strongly about removing the batch size I'm fine with doing that, too.

[jkemp101]

@thomasst Ship it