refactor: code host config watching
Created by: nicksnyder
similar to https://github.com/sourcegraph/sourcegraph/pull/1267/files except for all other code hosts
part of https://github.com/sourcegraph/sourcegraph/issues/914#issuecomment-440067022
Codehost configs will soon be fetched from the database. This means this code can no longer user conf.Watch.
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.