Add tests around hard network failures and node kicking.
Substantially improve HT interaction in node connection.

- Make almost everying visible to still-connecting nodes by introducing
  a maximum visibility lag, beyond which everything is considered
  public.

- Don't let still-connecting nodes service HtRange requests, because
  they can't possibly provide full answers, and provide a mechanism for
  them to indicate such.
Implement and test praef_system_disconnect().
Agree with majority when node gains DENY.