TTL for jobs

Time to live for jobs. I had a case where I wanted to not process jobs older than a certain date. Easiest solution was to add a ttl attribute to the job and if set expire the job in redis. The job will still be in the list of jobs but rq will just discard it. A job had already a `get_ttl` method that returned the result ttl. I renamed it to `get_result_ttl` and added a `get_ttl` that returns the job ttl.
Well yes, right now it is, but there's no reason you couldn't call it here too. :)

My thought was that it makes sense to call `cleanup()` here instead of explicitly calling `connection.expire()`. It's a small thing, but it deduplicates code that does essentially the same thing - i.e. configuring a job hash to expire in Redis.