Support statistics runtime of finished job

[yyaadet]

I add a new command named rq_runtime to display runtime of functions. And inset a new column ended in the jobs list page.

[selwin]

@yyaadet I cherry-picked your commit that shows job.ended_at in this https://github.com/rq/django-rq/pull/557 . I'm not sure about iterating through all jobs in FinishedJobRegistry and averaging it on the fly.

If we were to have registry level statistics, I think we should build it into RQ proper, like Worker statistics.