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
* Plumbs the limit through all backends ensuring they don't do
unnecessary work.
* Don't arbitrarily limit at the backend with hardcoded defaults. The
limit will be set by the caller.
Prior to this change the code on recursive in some backends would do 10k
results from the database and then only return the first 1k (throwing
out 9k of them).
Prior to this change some backends had no limit at all (e.g. redis).
Change-Id: I1f327eefe095776d123dd11362cd00994c22efdf
