Add activate and deactivate impl for RabbitMQSpout.

Implement activate() and deactivate() method in RabbitMQSpout.
For activate method, we need to initialise new RabbitMQConsumer because we can't reuse the closed RabbitMQConsumer. 

I've tested it with a topology in Storm cluster and it works. However, since there is no unit test in this project, I don't create the corresponding unit tests for this change.