Set root job information for purge jobs to avoid excess work

* In particular, this is useful if page_touched happens to be bumped after the job is made for some pages (for whatever reason). It also useful if we put in jobs for files and redirects to them, as that could cause pages to be hit twice without this.

Change-Id: Id7f3151f764cd608acc0007561474c9bed5181a9
Merge "Set root job information for purge jobs to avoid excess work"