feat(swarm): Make executor for connection tasks explicit (libp2p#3097)

Previously, the executor for connection tasks silently defaulted to a `futures::executor::ThreadPool`. This causes issues such as libp2p#2230.

With this patch, we force the user to choose, which executor they want to run the connection tasks on which results in overall simpler API with less footguns.

Closes libp2p#3068.

examples/autonat_client.rs

@@ -67,7 +67,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
 
     let behaviour = Behaviour::new(local_key.public());
 
-    let mut swarm = Swarm::new(transport, behaviour, local_peer_id);
+    let mut swarm = Swarm::with_async_std_executor(transport, behaviour, local_peer_id);
     swarm.listen_on(
         Multiaddr::empty()
             .with(Protocol::Ip4(Ipv4Addr::UNSPECIFIED))

examples/autonat_server.rs

@@ -57,7 +57,7 @@ async fn main() -> Result<(), Box<dyn Error>> {
 
     let behaviour = Behaviour::new(local_key.public());
 
-    let mut swarm = Swarm::new(transport, behaviour, local_peer_id);
+    let mut swarm = Swarm::with_async_std_executor(transport, behaviour, local_peer_id);
     swarm.listen_on(
         Multiaddr::empty()
             .with(Protocol::Ip4(Ipv4Addr::UNSPECIFIED))

tests/test_client.rs

@@ -40,7 +40,7 @@ async fn init_swarm(config: Config) -> Swarm<Behaviour> {
     let local_id = PeerId::from_public_key(&keypair.public());
     let transport = development_transport(keypair).await.unwrap();
     let behaviour = Behaviour::new(local_id, config);
-    Swarm::new(transport, behaviour, local_id)
+    Swarm::with_async_std_executor(transport, behaviour, local_id)
 }
 
 async fn spawn_server(kill: oneshot::Receiver<()>) -> (PeerId, Multiaddr) {

tests/test_server.rs

@@ -39,7 +39,7 @@ async fn init_swarm(config: Config) -> Swarm<Behaviour> {
     let local_id = PeerId::from_public_key(&keypair.public());
     let transport = development_transport(keypair).await.unwrap();
     let behaviour = Behaviour::new(local_id, config);
-    Swarm::new(transport, behaviour, local_id)
+    Swarm::with_async_std_executor(transport, behaviour, local_id)
 }
 
 async fn init_server(config: Option<Config>) -> (Swarm<Behaviour>, PeerId, Multiaddr) {