RoutingKeyGenerator:

  - Move from core to RouterKeyGenerator in router.jar
  - Leave RoutingKeyGenerator as a simple abstract class
  - DatabaseEntry now uses timestamp instead of mod data
    to determine if mod data has changed. Don't expose
    mod data to DatabaseEntry any more.
  - I2PAppContext.routingKeyGenerator() now returns null;
    you must be in RouterContext to get a generator.