Enable reading server pushes in batches

[mzimbres]

At the moment the asio::channel used to communicate server pushes to the user via async_receive has size 0, which is inefficient under heavy load. Given that the generic_response can contain multiple responses and pushes, there would be no problem is making the channel size configurable by the user.

The implementation should also use the try_send/receive functions to avoid suspension when possible.

It should be also documented that a generic_response might contain multiple server pushes for channels with size != 0 and how to iterate over them, which at the moment must be done by checking nodes with depth 0. In the future we will add a nicer way of iterating over individual messages in a response.