Directory node reconnect attempt slowdowns vs Internet / Tor downtime

[kristapsk]

In #1436 exponentional delay was added to reconnect attempts to dnodes with idea that if individual ones are not working at the moment to not do too many useless reconnect attempts. That works well except if Internet or Tor in general goes down for prolonged amount of time. In that case you end up with yield generator not doing reconnect attempts to any dnodes for a long time when you get back online. We should try to detect somehow is there problem with individual dnode or Internet or Tor connection in general.