jc_utils: introduce running_tasks function.

This function returns the list of the running tasks in a certain stage.
disco.erl: introduce the enum/2 helper function.

This function is just like enum/1 but can start sequencing from any number (not
just 0).
disco_worker: use the update function instead of directly sending inputs to the worker.

When the worker_runtime decides that it should not respond to an input message
immediately, the disco_worker can avoid consuming that INPUT message from the
worker and keep it pending.

That means, when the input is ready, disco_worker can just call the update
functio.  The INPUT message is going to be consumed but this time, there are
some inputs available which will bee sent to the worker.
worker_input: implement the add_inputs function.

In the current implementation, the inputs received by worker_input may contain
duplicates and have to be taken care of.  Moreover, the sequence numbers given
to the inputs are important.  Disco should give brand new sequence numbers to
the new inputs.
job_coordinator: a couple of things.