This reverts merge request !802

So we don't flood our postgres logs with errors. Should also make things
slightly faster.

This was a dirty regex replace which worked on my server

Added tests.

(calling `Instances.set_reachable/1` only if instance had `unreachable_since`,
calling `Instances.set_unreachable/1` only if instance had nil `unreachable_since`).

This reverts merge request !733

that is the only change from nodeinfo 2.0 to 2.1

also this makes the nodeinfo tests use 2.1.json instead of 2.0.json

This reverts merge request !682

[#534] Made federation push sender be determined basing on content instead of `referer` header. Updated tests.