relay circuitv2: relaymanager starts new relays without closing older ones

[sukunrt]

While debugging #2046 , we found out that autonat would emit events when the observed address has changed even though the node reachability hasn't changed.

I found out that the LocalProtocolsUpdated event was coming from RelayManager.reachabilityChanged which starts a new relay if the observed event is reachability.Public
https://github.com/libp2p/go-libp2p/blob/master/p2p/host/relaysvc/relay.go#L65-L85

This is incorrect. It should only start a new relay if the previous state was not public or at least close any previous running relays and then start a new one.

cc: @marten-seemann