babeld could be in bad state, or it could be incompatible (too old or too new).

- getBootstrapPeer was stuck as long as there was no other request being served
- registry crashed when re6stnet is stopped

Code 4 was reused by mistake for 'kill' messages.

Co-authored-by: Julien Muchembled <jm@nexedi.com>

Here, it's simpler and safer. We will also want to have private methods that
don't start with an underscore.

Certificates are deleted 30 days after they get invalid,
so that unused prefixes can be reallocated.

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.

We'll have to revive UDP because we experienced congestion with TCP.
This should make UDP efficient in good environment.
MTU discovery is required however to enable UDP by default.

This fixes up commit e3781aff
(Reduce probability of dying when the system lacks memory").

- new -V/--version command line option
- protocol extended to get the version of any node in the network,
  which will allow to track those running an old version of re6st

It's already in 're6stnet_subnet' and it's useful to bind to 're6stnet_ip'
without having to edit it first.

256 * (1 - (1-42/256.)**(15/4.)) ≈ 125
where:
- 42 is default rtt-decay
- 4 is default hello in babeld
- 15 is default hello in re6st

This makes sure re6st does not stop working because a connection is stuck.