Delete finished job in show action

[icapuccio]

Currently, the async_request_jobs table gets bigger and bigger with each async request. Since the client stops asking for a job once it is finished, it would be nice to have an automatic depuration in the table.

Another issue with the current behavior is that when you queue a job with a sensible parameter (like a password) it remains accessible for anyone that query the table.

I believe that this whole issue could be solved by performing a job destroy inside JobsController#show --> render_finished_job(job)

[petaalcala]

#20