We should launch a reachability test immediately if a router changes IP or port

When reviewing my patch for bug legacy/trac#911 (moved), Sebastian noted:

in dirserv_should_launch_reachability_test() shouldn't we test for difference in port or ip, and do the test immediately?

This is indeed so. We should do this once legacy/trac#911 (moved) is merged.

Marking for 0.2.2.x-final, since it's trivial.