Use the Amazon SQS Bulk Fetch API to implement the Kombu prefetch_count.

This patch implements a new private method on the SQS Channel
object named _get_bulk(). This method calls out to the QoS object
to get a number of estimated messages its allowed to retrieve
from SQS at any given moment, and then pulls them down and returns
a list of these messages.

An updated drain_events() method then stores these messages in
a local cache object, and pops them off one by one as the drain_events()
method is called by the Transport.drain_channel() method.