Exclude exit-port statistics from extra-info descriptors

Very fast relays generate exit-port statistics that require 40-45 KB. These statistics can easily make the relay generate an extra-info descriptor that exceeds the 50 KB upload limit. See #2183 (moved).

Should we exclude exit-port statistics from extra-info descriptors? We could instead append them to a local file, so that they are still available if someone wants them.

I'd like to hear Steven's opinion here, because he was the reason why we implemented these stats a year ago.