A very basic check which every day tries to connect to every known instance. If the connection fails or returns something different than HTTP 200, the instance is marked as dead and no federation activities will be sent to it.

This implementation is really basic, there can be false positives if an instance is temporarily down or unreachable during the check. It also rechecks all known instances every day, even if they have been down for years. Nevertheless it should be a major improvement, we can add more sophisticated checks later.

Still need to fix two problems mentioned in comments.

Originally posted by Nutomic in #3427