Shorten and summarize fallback directory irc notifications

Adjusting the irc notifications per teor's request...

  https://trac.torproject.org/projects/tor/ticket/30940

In particiular notifications now begin with a count summary (like emails), and
present at most four relays. For example...

  [fallback-directories] 157/157 (100%) fallback directories have become slow or unresponsive...
  [fallback-directories] 8FA37B93397015B2BC5A525C908485260BE9F422 => ORPort is unreachable (81.7.11.96:9001)
  [fallback-directories] 36B9E7AC1E36B62A9D6F330ABEB6012BA7F0D400 => ORPort is unreachable (37.187.22.87:9001)
  [fallback-directories] 230A8B2A8BA861210D9B4BA97745AEC217A94207 => ORPort is unreachable (163.172.176.167:443)
  [fallback-directories] 823AA81E277F366505545522CEDC2F529CE4DC3F => ORPort is unreachable (192.160.102.164:9001)
  [fallback-directories] ... and 153 more