this will allow us to inspect the type of `db.Driver()` on *sql.DB
connections to correctly differentiate between pg and crdb conns.
as a bonus, this moves all concerns about when to replace "cockroach://"
with "postgres://" out of view, letting the thin shim "driver" take care
of that.
Change-Id: Ib24103ab7c508231e681f89a7321b623e4e125e9
