DB Backend: remove `basestore.NewWithDB`

Created by: camdencheek

This replaces the last instances of basestore.NewWithDB() and removes the function. Instead of constructing stores from a dbutil.DB interface, now we construct stores using other shareable handles or from transactable handles directly.

Depends on https://github.com/sourcegraph/sourcegraph/pull/36924 (sorry for the weird dependency. I had a couple of branches going. Will rebase once that's merged)

Test plan

Unit tests.