Investigate solutions to stop luigi start_start task when we stop celery.

Since we are invoking luigi in subprocess, when we try to stop celery, the task waits that luigi completes.

We can't really call luigi directly, becuse celery worker is running as a daemon, and luigi.build() uses multiprocessing, which leads to AssertionError: daemonic processes are not allowed to have children.

See !44 (closed)

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