The fix to mark an interface as "up" and indicate its MTU was
useful for machines with a single client, because OpenVPN would fail
to configure them this way in OpenVPN 2.3. It has been fixed in 2.4
so the fix has been removed.

/reviewed-on !14

This fixes the following error:

  TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'
  Traceback (most recent call last):
    File "/usr/sbin/re6stnet", line 438, in main
      tunnel_manager.handleTunnelEvent(read_pipe.readline())
    File "/usr/lib/python2.7/dist-packages/re6st/tunnel.py", line 389, in handleTunnelEvent
      m(*args)
    File "/usr/lib/python2.7/dist-packages/re6st/tunnel.py", line 412, in _ovpn_route_up
      self._connection_dict[prefix].connected()
    File "/usr/lib/python2.7/dist-packages/re6st/tunnel.py", line 76, in connected
      i = self._retry - 1

What happened is probably that a route_up notification was received just before
killing/recreating the connection for the same node, and then process twice
the same OpenVPN notification: in this case, the first was for a previous
connection and should have been ignored.

For the moment, this does nothing since it should always be 1500.

The previous broadcast model is replaced by a query-response one.
During normal operation, the cache of peers is not used anymore to select
peers to connect to. It now only used for bootstrapping and avoid querying
an already known address.