gitserver: Write a repo migration worker

Created by: asdine

Write a worker that migrates repositories from one gitserver instance to another.

• The worker reads from jobs from the database and executes them concurrently.
• Each job represents:
  • a repository name
  • the source instance
  • the destination instance
  • if the source repository must be deleted after the clone
• The number of concurrent workers must be configurable and defaults to 5.
• The number of concurrent migrations per destination instance must be configurable and defaults to 1.