DB Backend: convert most handles to typed handles

Created by: camdencheek

This builds on some of my previous PRs and converts more TransactableHandle instances to use the well-typed handles. After this PR, there is exactly one usage remaining of NewHandleWithUntypedDB(), which is used by database.NewUntypedDB(), which is still used in a handful of places. I will work on converting the rest of the call sites in the next PR, but this one just handles a couple of easier instances.

Stacked on https://github.com/sourcegraph/sourcegraph/pull/37155

Test plan

No change in semantics. Relying on unit tests.