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
for storj-sim to work, we need to avoid schemas in cockroach urls
so we have storj-sim create namespaced databases instead of schemas
and we have the migrate command create the database in the same way
that it would create a schema for postgres. then it works!
a follow up commit will move the creation of the database/schemas
into storj-sim's setup step so that we can avoid doing these icky
creations during normal migration calls. it will also make the
pointerdb have an explicit call to migrate instead of just doing
it every time it's opened.
Change-Id: If69ef5cb96b6866b0438c761bd445afb3597ae5f
