disco_worker: Send the pid of the slave worker instead of the worker process on master.
job_coordinator: use the task_id in the task_started message.
worker_runtime: Add a producer-consumer structure.

The runtime used to tell the worker that there is no more inputs in all of the
cases.  In the new model, the runtime will tell the worker that there is more
inputs in case there is more inputs to come.

At the moemnt, the runtime has all of the inputs when the task starts but this
will change with the concurrent stages.