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.