Implement Go routine which serves UpdateControlMap DBus endpoint

Acceptance criteria:

• Implement a new DBus endpoint:
  • Connection name: io.mender.UpdateManager
  • Object path: /io/mender/UpdateManager
  • Interface: io.mender.Update1
  • Method name: SetUpdateControlMap
  • Parameters:
    • update_control_map (string): JSON data (exact format described in MEN-4546, but assume it has id and priority keys)
  • Returns:
    • refresh_timeout (int): Seconds before the request should be refreshed. This should be UpdateControlMapExpirationTimeSeconds / 2.
• Endpoint must be served from a new Go routine separate from the main thread. This is to keep responsiveness even during an update.
  • Do not reuse the Auth Go routine, we may split this into a separate binary later, so the code should not be mixed.
• The id and priority fields together form the primary key for the map. All maps with unique keys are stored, but identical keys overwrite each other. For example:

  [id: 123456, priority: 0] + [id: abcdef, priority: 0] = store both
  [id: 123456, priority: 0] + [id: abcdef, priority: 1] = store both
  [id: 123456, priority: 0] + [id: 123456, priority: 1] = store both
  [id: 123456, priority: 0] + [id: 123456, priority: 0] = overwrite
  

• All accesses to read and write the maps must be thread safe (mutex probably).
• The client facing API should be SetMapValue (or similar name)
• Unit tests
• Feature branch: feature-dbus_update_control