This logic collects a list of known relays by testing every new connection.
It exists so we can dial /p2p-circuit/p2p/QmFoobar addresses (circuit addresses
that don't specify the relay). However, this kind of address is useless outside
of basic demos as a random relay is practically guaranteed to not be connected
to the target peer. Picking a random relay to connect to some peer is almost
_never_ the desired behavior.
Unfortunately, we don't currently have any way to pick out good relays from bad.
That means we keep searching, trying bad relays, searching some more, trying
_the same relays_, etc. until we randomly find 3 good stable relays. In
practice, this means we just keep searching forever and keep thrashing the DHT.
see https://github.com/libp2p/go-libp2p/issues/694
