IVS-356 - Terminate subprocesses in celery after timeout (PAUSED)

[aothms]

First attempt combing two approaches:

• I think as @rw-bsi suggested, use Popen and terminate when catching celery's injected soft timeout exception
• but also a generic vacuum of any lingering process (not sure if necessary - it's what I started with)
• we could also choose either one of the approaches I think

Tested with:

CELERY_TASK_SOFT_TIME_LIMIT = 4
CELERY_TASK_TIME_LIMIT = 6

[Ghesselink]

Superceded by #241