Spawn changeset syncer per code host

Created by: eseliger

Previously, it was per external service, but then when determining the token it wasn't neccessarily using the same external service, so this coupling was unneeded. Now we only spawn one per code host, which should help get down the syncer count a lot. Also, once we don't use the tokens from the external services anymore, this will come in handy as we don't need to rely on external services for the syncer at all at that point.

Once users can add external services, this would also have caused an explosion in syncers spawned. Good for future-proofing, I guess 🤷‍♂️