AMQP RPC, use direct-reply-to

[carlhoerberg]

The RPC backend should use the very fast direct-reply feature in RabbitMQ: https://www.rabbitmq.com/direct-reply-to.html instead of creating a new queue for each result. Especially for clustered server creating a deleting queues is a very slow process, crippling the RPC throughput to a few requests/s.