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
dbschema.Query is used only for testing and sqlite,
so this won't cause us problems in production.
Change-Id: Ib296a7daf161a9d3de23a7dfdc4f505d47ac4a37
