feat(ping): don't close connections upon failures

Previously, the `libp2p-ping` module came with a policy to close a connection after X failed pings. This is only one of many possible policies on how users would want to do connection management.

We remove this policy without a replacement. If users wish to restore this functionality, they can easily implement such policy themselves: The default value of `max_failures` was 1. To restore the previous functionality users can simply close the connection upon the first received ping error.

In this same patch, we also simplify the API of `ping::Event` by removing the layer of `ping::Success` and instead reporting the RTT to the peer directly.

Related: #3591.

Pull-Request: #3947.

examples/rendezvous/src/bin/rzv-discover.rs

@@ -105,7 +105,8 @@ async fn main() {
                     }
                     SwarmEvent::Behaviour(MyBehaviourEvent::Ping(ping::Event {
                         peer,
-                        result: Ok(ping::Success::Ping { rtt }),
+                        result: Ok(rtt),
+                        ..
                     })) if peer != rendezvous_point => {
                         log::info!("Ping to {} is {}ms", peer, rtt.as_millis())
                     }

examples/rendezvous/src/bin/rzv-identify.rs

@@ -106,7 +106,8 @@ async fn main() {
             }
             SwarmEvent::Behaviour(MyBehaviourEvent::Ping(ping::Event {
                 peer,
-                result: Ok(ping::Success::Ping { rtt }),
+                result: Ok(rtt),
+                ..
             })) if peer != rendezvous_point => {
                 log::info!("Ping to {} is {}ms", peer, rtt.as_millis())
             }

examples/rendezvous/src/bin/rzv-register.rs

@@ -104,7 +104,8 @@ async fn main() {
             }
             SwarmEvent::Behaviour(MyBehaviourEvent::Ping(ping::Event {
                 peer,
-                result: Ok(ping::Success::Ping { rtt }),
+                result: Ok(rtt),
+                ..
             })) if peer != rendezvous_point => {
                 log::info!("Ping to {} is {}ms", peer, rtt.as_millis())
             }