Support "max batch size"

[jaslong]

Analogous to graphql/dataloader#42

Very useful with the KV source, where the load function is calling batch APIs with max batch sizes.

I may be able to help implement this. If I were to implement to this, should the "max batch size" functionality be in the Dataloader or KV level? I think it makes sense at the Dataloader level to allow any source to use this functionality.

[benwilson512]

I think this depends on the source, since whether this limit can even be used depends on the source. PR welcome.