Worker-side job status check should not use the database

[Muharem Hrnjadovic]

Presently Job.is_job_completed() obtains the job status from the
postgres database.
This introduces a new and unnecessary dependency. The job status
check should be based on data in the redis key-value-store instead.

[Lars Butler (lars-butler)]

The `check_job_status` function which calls this bit of code is everywhere in our task code; it's called as the first statement of each task. We should consider refactoring `check_job_status` as a decorator to make things cleaner.

[Muharem Hrnjadovic (al-maisan)]

We may want to re-architect the system so that the workers compute results *and* write them to the database. This should give us better throughput.

In light of that it's fine for the workers to access the postgres database.

[Lars Butler (lars-butler)]

No need to fix. The workers do a lot of database interaction anyway. This is fine.