Recursive dialing can block

[Stebalien]

If two peers advertise each other as, e.g., relays and a third peer tries to dial both of these peers at the same time, the dial will bock.

• Thread 1: Start dial to peer A
• Thread 2: Start dial to peer B
• Thread 1: Try to dial the relay (B) for peer A.
• Thread 2: Try to dial the relay (A) for peer B.

Now, given that we allow 8 parallel dials per peer (default), this shouldn't be the end of the world. In most cases, we'll also try dialing peer A and B directly, if we have the appropriate addresses.

Also note, each peer will timeout dialing the relay after 60 seconds because that's the default transport dial timeout.

However, we should still provide a solution for this.

Initially reported in libp2p/go-libp2p-swarm#161.