[rabbitmq] Ensure at most once delivery.

[jaredallard]

Right now it's possible for two jobs of the same media to be processed at the same time, it is also possible for two instances to get the same job (unack -> broker dies -> unack on new + old). We should have a locking system that acts on both type id and job id.

This would be done in the tritonmedia/core repository, in the ampq.js file. This is used by all applications for AMQP connections.