wip: refactor: codehost config watching

Created by: nicksnyder

part of https://github.com/sourcegraph/sourcegraph/issues/914#issuecomment-440067022

In a followup PR, codehost configs will be fetched from the external services table in the database. This means this code can no longer user conf.Watch (since the code host config will no longer live in the config).

This PR preemptively refactors the code to not depend on conf.Watch. It also includes a minor improvement to not restart workers unless the relevant config actually changes.