Add completed task pruning feature

[furosa]

Add prune_completed_tasks method. This method deletes from the task_queue postgres table all tasks completed more than the specified hours ago; tasks being processed, not started or completed recently will stay in the table.

[jacopofar]

Thanks! I like it, but I'd prefer to use seconds for the interval to keep consistency with the lease timeout, and current_timestamp instead of now() as it's standard SQL (it does the same thing, it's just nicer to see for me).

so, basically:

                       AND completed_at < current_timestamp - CAST(
                            %s || ' seconds' AS INTERVAL);

and the corresponding change in the documentation

[jacopofar]

There's some linting issue

[furosa]

There's some linting issue

Thanks for the heads up, it should be fixed!