Node relationship type added

Checking whether a connection is allowed can now be handled more
fine-grained than with a Bool: self/downstream/upstream/unrelated can be
distinguished.
Network semi-working

Tested running a network with 8 nodes and minimum 3 neighbours in each
direction.

- Discovery works, even though the bootstrapping server is very crude.
- Starting the network leaves many nodes in perma-search mode.
  Restarting them sometimes help to stabilize the network.
- All nodes seem to have more outgoing than incoming connections, which
  should be impossible. Haven't found the reason for this yet. As a
  result, the downstream pool fills up much easier than the upstream
  one.
Network working reasonably well

Housekeeping in the previous commit was incorrect: it updated the
timestamps of all non-timeout nodes - which means that as long as
housekeeping runs faster than timing out takes, all timestamps will
always be updated.