Improve route cleanup

[damb]

Cleaning up routes is not automatized, yet. That is, once a route has been harvested it remains in the routing definitions without being removed. Clients of eidaws-federator do not notice this fact, however, it

• forces eidaws-federator to perform endpoint requests just to receive no data (i.e. HTTP 204|404 status codes) and therefore it
• degenerates the performance.

[damb]

The implementation should be able to handle the case when a datacenter is unavailable during the harvesting procedure.