Make autoconnect try to heal network splits.

When we have less than three connections, we greedily try to connect to any
viable node. However, once we have three connections, try to connect to
nodes that we know of but that aren't reachable.

We also make sure that if there are 100 reachable nodes, and 1 unreachable
one, that not all 100 reachable nodes try to connect to the unreachable
at the same time.