Refactor dataloaders into a general-purpose batching struct

[radazen]

What's new?

I occasionally find myself wanting a reusable batcher that can do what dataloaders do: open a batch with a max size and timeout, wait for requests to come in, submit all the queued up requests when the batch closes, etc. I've reused this pattern in our push notifications handling, and wanted to use it again for Kafka event handling.

This PR refactors the Dataloader[TKey, TResult] type to be a thin wrapper around a reusable batch.Batcher[TParam, TResult] class that can be reused in any other context where we want batching.

The most notable change is that the Load functions have been renamed to Do, since we don't really know if a generic batch is "loading" things or just executing commands and receiving a result status. The Dataloader wrapper maintains the existing Load function names, since those feel appropriate and idiomatic for a dataloader.