Implement running without route table

[LeeSmet]

Currently a node always has the full route table. This might not always be desirable, especially for nodes with limited memory. After #94, we can essentially run with only a minimal list of routes, based on a LRU policy. When a route is needed for a subnet which is not locally present, peers are queried with a route request, and the best route is used as it comes in. This will incur some startup latency, but avoid the memory usage of the route table. A startup parameter could be used to specify how long an idle route should be kept.

[LeeSmet]

One major issue is receiving data, since we don't have a route table we will likely not have the keys needed to decrypt either. We could employ a similar strategy and send a route request to neighbours when we receive a packet from a subnet we don't have the decryption key for.

[delandtj]

Hmm .. I don't see that as a big feature, but the framwork to do so might come in handy when we want to split routing tables in bigger chunks through concentrators (a bit like IXes)