GossipRouter: make GraalVM-compliant

Currently, GossipRouter starts threads in the constructor. This prohibits it to run under GraalVM as a native image, as threads cannot be started at build time.

Create a separate method init(), which is called after the constructor and after all attributes have been set, but before start().

Alternative: use a builder:

router=GossipRouter.builder().setXX().build(); // creates server
router.start();