chore: add simple h2 benchmark (#762)

This PR adds a simple benchmark to measure perf improvement changes. E.g., a potential fix for this issue: #531

The benchmark is simple: have a client send `100_000` requests to a server and wait for a response. 

Output:
```
cargo bench
H2 running in current-thread runtime at 127.0.0.1:5928:
Overall: 353ms.
Fastest: 91ms
Slowest: 315ms
Avg    : 249ms
H2 running in multi-thread runtime at 127.0.0.1:5929:
Overall: 533ms.
Fastest: 88ms
Slowest: 511ms
Avg    : 456ms
```

Cargo.toml

@@ -71,3 +71,7 @@ webpki-roots = "0.25"
 
 [package.metadata.docs.rs]
 features = ["stream"]
+
+[[bench]]
+name = "main"
+harness = false

benches/main.rs

@@ -0,0 +1,148 @@
+use bytes::Bytes;
+use h2::{
+    client,
+    server::{self, SendResponse},
+    RecvStream,
+};
+use http::Request;
+
+use std::{
+    error::Error,
+    time::{Duration, Instant},
+};
+
+use tokio::net::{TcpListener, TcpStream};
+
+const NUM_REQUESTS_TO_SEND: usize = 100_000;
+
+// The actual server.
+async fn server(addr: &str) -> Result<(), Box<dyn Error + Send + Sync>> {
+    let listener = TcpListener::bind(addr).await?;
+
+    loop {
+        if let Ok((socket, _peer_addr)) = listener.accept().await {
+            tokio::spawn(async move {
+                if let Err(e) = serve(socket).await {
+                    println!("  -> err={:?}", e);
+                }
+            });
+        }
+    }
+}
+
+async fn serve(socket: TcpStream) -> Result<(), Box<dyn Error + Send + Sync>> {
+    let mut connection = server::handshake(socket).await?;
+    while let Some(result) = connection.accept().await {
+        let (request, respond) = result?;
+        tokio::spawn(async move {
+            if let Err(e) = handle_request(request, respond).await {
+                println!("error while handling request: {}", e);
+            }
+        });
+    }
+    Ok(())
+}
+
+async fn handle_request(
+    mut request: Request<RecvStream>,
+    mut respond: SendResponse<Bytes>,
+) -> Result<(), Box<dyn Error + Send + Sync>> {
+    let body = request.body_mut();
+    while let Some(data) = body.data().await {
+        let data = data?;
+        let _ = body.flow_control().release_capacity(data.len());
+    }
+    let response = http::Response::new(());
+    let mut send = respond.send_response(response, false)?;
+    send.send_data(Bytes::from_static(b"pong"), true)?;
+
+    Ok(())
+}
+
+// The benchmark
+async fn send_requests(addr: &str) -> Result<(), Box<dyn Error>> {
+    let tcp = loop {
+        let Ok(tcp) = TcpStream::connect(addr).await else {
+            continue;
+        };
+        break tcp;
+    };
+    let (client, h2) = client::handshake(tcp).await?;
+    // Spawn a task to run the conn...
+    tokio::spawn(async move {
+        if let Err(e) = h2.await {
+            println!("GOT ERR={:?}", e);
+        }
+    });
+
+    let mut handles = Vec::with_capacity(NUM_REQUESTS_TO_SEND);
+    for _i in 0..NUM_REQUESTS_TO_SEND {
+        let mut client = client.clone();
+        let task = tokio::spawn(async move {
+            let request = Request::builder().body(()).unwrap();
+
+            let instant = Instant::now();
+            let (response, _) = client.send_request(request, true).unwrap();
+            let response = response.await.unwrap();
+            let mut body = response.into_body();
+            while let Some(_chunk) = body.data().await {}
+            instant.elapsed()
+        });
+        handles.push(task);
+    }
+
+    let instant = Instant::now();
+    let mut result = Vec::with_capacity(NUM_REQUESTS_TO_SEND);
+    for handle in handles {
+        result.push(handle.await.unwrap());
+    }
+    let mut sum = Duration::new(0, 0);
+    for r in result.iter() {
+        sum = sum.checked_add(*r).unwrap();
+    }
+
+    println!("Overall: {}ms.", instant.elapsed().as_millis());
+    println!("Fastest: {}ms", result.iter().min().unwrap().as_millis());
+    println!("Slowest: {}ms", result.iter().max().unwrap().as_millis());
+    println!(
+        "Avg    : {}ms",
+        sum.div_f64(NUM_REQUESTS_TO_SEND as f64).as_millis()
+    );
+    Ok(())
+}
+
+fn main() {
+    let _ = env_logger::try_init();
+    let addr = "127.0.0.1:5928";
+    println!("H2 running in current-thread runtime at {addr}:");
+    std::thread::spawn(|| {
+        let rt = tokio::runtime::Builder::new_current_thread()
+            .enable_all()
+            .build()
+            .unwrap();
+        rt.block_on(server(addr)).unwrap();
+    });
+
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    rt.block_on(send_requests(addr)).unwrap();
+
+    let addr = "127.0.0.1:5929";
+    println!("H2 running in multi-thread runtime at {addr}:");
+    std::thread::spawn(|| {
+        let rt = tokio::runtime::Builder::new_multi_thread()
+            .worker_threads(4)
+            .enable_all()
+            .build()
+            .unwrap();
+        rt.block_on(server(addr)).unwrap();
+    });
+
+    let rt = tokio::runtime::Builder::new_current_thread()
+        .enable_all()
+        .build()
+        .unwrap();
+    rt.block_on(send_requests(addr)).unwrap();
+}