RFC 211: Repo updater should sync external services separately

Created by: ryanslade

repo-updater should sync each external service separately, backing off when no change has occurred.

It should use the workerutil package to schedule jobs that can be process by n syncers concurrently.

We should ensure that:

• Private code cannot by synced for user owned external services
• During long syncs, new repos appear in our db ASAP
• Sync performance is no worse than the previous syncer