Reach test

Fix a bug in my keyspace-hole-filling code which could allow a hole to reopen and go unnoticed (basically, the address it was looking up to check/fill a hole didn't guarantee that a valid next hop was known for all possible addresses in the same area of keyspace).

Added a similar function to the global maint cycle. In kademlia terms, if we had real buckets, it's approximately the part where you pick a random bucket and ping the node at the top. It actually uses a search instead, to also find more nodes while we're at it (like k-buckets for k>1, but without actually picking a constant k).