Replace all the remaining uses of sql.DB with tagsql.DB to
fix issues with context cancellation.
Introduce tagsql.Open which helps to get rid of all tagsql.Wrap-s.
Use tagsql in cockroachkv and postgreskv.
Change-Id: I8946d203341cb85a25976896fc7881e1f704e779
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
first, so that they all work the same way, because it's getting
complicated, and second, so that we can do the appropriate thing
instead of CREATE SCHEMA for cockroachdb.
Change-Id: I27fbaeeb6223a3e06d97bcf692a2d014b31465f7
