Mysql retry

This adds a retry block around calls made with a dhub.  This wraps both ``jobs.py`` and ``refdata.py`` calls to the datasource dhub.

I decided to do this in the ``utils.py`` file because ``jobs`` and ``refdata`` don't have a common ancestor.  And this seemed very simple.  I think this was mdoglio's initial take on the right approach.  I tried another where I modified the ``MySQL`` object in datasource directly, but it became too complicated and hacky feeling.  (monkeypatch, etc...)

This is now on stage.

**Also**: I removed some dead code re: ``get_row_by_id`` that's not used anywhere anymore.
I guess this doesn't belong strictly to this PR?