Increase how long supervisor waits for workers to stop.

This is set as 5 days for now because we have seen "grade_course" tasks that
take about 4 days to complete for large classes and if we shutdown before
they are done we end up in a bad state in the database.
Merge pull request #1410 from edx/feanil/increase_worker_timeout

Increase how long supervisor waits for workers to stop.