[rackspace] Queues: makes block optional when dequeuing

Currently when you want to dequeue a message you always have to pass a block, even if you have no reasons for one. I've found my self passing empty blocks all the time, when I just want to dequeue a message.

This PR makes a simple modification to make the block optional. This way you don't need  to pass an empty block when you don't need one. No side effects and backward compatible.