add batch processing for amqp consumers

[gfmio]

Right now, amqp consumers only support handling one message at a time, while kafka consumers have a batch_size parameter.

Is it feasible to add batch processing for amqp consumers? Adapting the typing is slightly annoying, because the current handler function type expects a single message, but it's doable with overloading.

Or is there a reason I'm missing why the amqp consumer is only handler one message at a time right now?