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
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
