Deliver message batches

Currently, message batches are sent up tp the channel, which delivers its messages one-by-one, e.g. by calling the Receiver.receive(Message msg) callback.

Another callback Receiver.receive(MessageBatch batch) should be added (also to RpcDispatcher), to deliver a batch of messages at once. The default implementation is to pass messages up one-by-one.

This should result in performance increase, as the application can handle multiple messages at once.