Previously, the blacklist would never be retried since we queued the
wrong list of addresses. New logic also drops peers from the blacklist
if they've been continuously retried without success for a while.
This will do one hop of additional graph crawl every time the update
interval fires. It will blacklist peers who are not consistently
available and over time should converge to the set of stable peers
suitable for DNS bootstrapping.
Conrado Gouvea
George Tankersley