repo-updater: More explicit single service syncing

Created by: ryanslade

Instead of triggering all sync jobs here: https://github.com/sourcegraph/sourcegraph/blob/7252febdda9cbccf5dd08780a30f0b4dc045b869/cmd/repo-updater/repoupdater/server.go#L344

We should instead enqueue just the external service that was requested. We can do this by creating a new store method.

We should also look into why we're using the trigger pattern in the first place as it may not be necessary.