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When network parameters are updated, their version is broadcasted
throughout the whole graph of nodes starting from the node running
the registry. This is of course not enough, at least because some
nodes may be stopped at that time.

A second mechanism already exists for late nodes: when 2 nodes starts
communicating between each other, they check their versions of the
network parameters. But it supposes that routing works well enough
between them.

Then comes HMAC, which can prevent 2 nodes from even handshaking and
group of nodes may remain stuck out of the network: this is the issue
that is addressed by this commit. 2 scenarios:
- no route to the registry node;
- there's a route to the registry node but not from it, which can only
  happen during the first phase of a HMAC change, i.e. when signing
  with the old key and accepting the new one:  because a node can't
  detect this case by itself, a new 'valid_until' network parameter
  to tell nodes that something is likely wrong after this date.
On either condition, a node checks for new HMAC keys from time to time.

This commit also drops a check for an old kernel bug.

See merge request !53