When 10.42.3.1/24 was configured on 'lo', the kernel accepted packets to
any ip of 10.42.3.0/24, instead of only 10.42.3.1

The old distutils way is not compatible with zc.recipe.egg in develop mode,
because egg_info does not provide any information about such scripts.

2.2.x branch has a patch[1] that renders tls_serial_* environment variables
in base 16, causing a ValueError exception in ovpn-server hook.

[1] https://github.com/OpenVPN/openvpn/commit/7d5e26cbb53e2700c966e6b6e815f0c824da8956

It doesn't matter anymore if there are many off nodes, because the registry
only queries the addresses of nodes that are in the routing table.

This does not increase of any packet because the size of certificate signature
only depends on the size of the certificate key.

With 512-bit hashes, it's still possible to use RSA keys as small as 768 bits.

There is no plan for a default ipv4 route.

This is useful because the default one is not always the same as the route
to the registry.

Before, arrows were filled dot. Now only the default one is filled.

This simplify network configuration a lot, and on recent kernels, this fixes
wrong source address for extra interfaces that already have a public IP.

Generating them takes a lot of time and there's no reason to do this by default.
We keep --dh option in 're6stnet' to not break existing configuration.

We consider using sockets to communicate with OpenVPN, via --management option.

db.py -> cache.py
PeerDB -> Cache
peers.db -> cache.db

And automatic renewal of existing certificates.

For the registry at least, we'll want to store integers
without having to convert to/from strings.

To upgrade 'registry.db':
- dump it to a file
- fix create table statements
- load it

Nodes will restart with an empty cache.

These modes are partly unified with the normal one by splitting TunnelManager.