feat(serve): Opt-in parallelism for `deno serve` (#24920)

Adds a `parallel` flag to `deno serve`. When present, we spawn multiple
workers to parallelize serving requests.


```bash
deno serve --parallel main.ts
```

Currently on linux we use `SO_REUSEPORT` and rely on the fact that the
kernel will distribute connections in a round-robin manner.

On mac and windows, we sort of emulate this by cloning the underlying
file descriptor and passing a handle to each worker. The connections
will not be guaranteed to be fairly distributed (and in practice almost
certainly won't be), but the distribution is still spread enough to
provide a significant performance increase.

---
(Run on an Macbook Pro with an M3 Max, serving `deno.com`

baseline::
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   239.78ms   13.56ms 330.54ms   79.12%
    Req/Sec   258.58     35.56   360.00     70.64%
  Latency Distribution
     50%  236.72ms
     75%  248.46ms
     90%  256.84ms
     99%  268.23ms
  15458 requests in 30.02s, 2.47GB read
Requests/sec:    514.89
Transfer/sec:     84.33MB
```

this PR (`with --parallel` flag)
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   117.40ms  142.84ms 590.45ms   79.07%
    Req/Sec     1.33k   175.19     1.77k    69.00%
  Latency Distribution
     50%   22.34ms
     75%  223.67ms
     90%  357.32ms
     99%  460.50ms
  79636 requests in 30.07s, 12.74GB read
Requests/sec:   2647.96
Transfer/sec:    433.71MB
```

4 files changed, 39 insertions, 14 deletions

diff --git a/ext/http/00_serve.ts b/ext/http/00_serve.ts
index 8ed1a1d04..9c6f80552 100644
--- a/ext/http/00_serve.ts
+++ b/ext/http/00_serve.ts
@@ -579,6 +579,8 @@ type RawServeOptions = {
   handler?: RawHandler;
 };
 
+const kLoadBalanced = Symbol("kLoadBalanced");
+
 function serve(arg1, arg2) {
   let options: RawServeOptions | undefined;
   let handler: RawHandler | undefined;
@@ -634,6 +636,7 @@ function serve(arg1, arg2) {
     hostname: options.hostname ?? "0.0.0.0",
     port: options.port ?? 8000,
     reusePort: options.reusePort ?? false,
+    loadBalanced: options[kLoadBalanced] ?? false,
   };
 
   if (options.certFile || options.keyFile) {
@@ -842,18 +845,25 @@ function registerDeclarativeServer(exports) {
         "Invalid type for fetch: must be a function with a single or no parameter",
       );
     }
-    return ({ servePort, serveHost }) => {
+    return ({ servePort, serveHost, serveIsMain, serveWorkerCount }) => {
       Deno.serve({
         port: servePort,
         hostname: serveHost,
+        [kLoadBalanced]: (serveIsMain && serveWorkerCount > 1) ||
+          (serveWorkerCount !== null),
         onListen: ({ port, hostname }) => {
-          console.debug(
-            `%cdeno serve%c: Listening on %chttp://${hostname}:${port}/%c`,
-            "color: green",
-            "color: inherit",
-            "color: yellow",
-            "color: inherit",
-          );
+          if (serveIsMain) {
+            const nThreads = serveWorkerCount > 1
+              ? ` with ${serveWorkerCount} threads`
+              : "";
+            console.debug(
+              `%cdeno serve%c: Listening on %chttp://${hostname}:${port}/%c${nThreads}`,
+              "color: green",
+              "color: inherit",
+              "color: yellow",
+              "color: inherit",
+            );
+          }
         },
         handler: (req) => {
           return exports.fetch(req);
diff --git a/ext/net/01_net.js b/ext/net/01_net.js
index 517ab127e..536f79bbf 100644
--- a/ext/net/01_net.js
+++ b/ext/net/01_net.js
@@ -531,10 +531,14 @@ const listenOptionApiName = Symbol("listenOptionApiName");
 function listen(args) {
   switch (args.transport ?? "tcp") {
     case "tcp": {
-      const { 0: rid, 1: addr } = op_net_listen_tcp({
-        hostname: args.hostname ?? "0.0.0.0",
-        port: Number(args.port),
-      }, args.reusePort);
+      const { 0: rid, 1: addr } = op_net_listen_tcp(
+        {
+          hostname: args.hostname ?? "0.0.0.0",
+          port: Number(args.port),
+        },
+        args.reusePort,
+        args.loadBalanced ?? false,
+      );
       addr.transport = "tcp";
       return new Listener(rid, addr);
     }
diff --git a/ext/net/ops.rs b/ext/net/ops.rs
index f28778d29..b74dc8d75 100644
--- a/ext/net/ops.rs
+++ b/ext/net/ops.rs
@@ -353,6 +353,7 @@ pub fn op_net_listen_tcp<NP>(
   state: &mut OpState,
   #[serde] addr: IpAddr,
   reuse_port: bool,
+  load_balanced: bool,
 ) -> Result<(ResourceId, IpAddr), AnyError>
 where
   NP: NetPermissions + 'static,
@@ -367,7 +368,11 @@ where
     .next()
     .ok_or_else(|| generic_error("No resolved address found"))?;
 
-  let listener = TcpListener::bind_direct(addr, reuse_port)?;
+  let listener = if load_balanced {
+    TcpListener::bind_load_balanced(addr)
+  } else {
+    TcpListener::bind_direct(addr, reuse_port)
+  }?;
   let local_addr = listener.local_addr()?;
   let listener_resource = NetworkListenerResource::new(listener);
   let rid = state.resource_table.add(listener_resource);
diff --git a/ext/net/ops_tls.rs b/ext/net/ops_tls.rs
index a2a27c4ad..8483e7e66 100644
--- a/ext/net/ops_tls.rs
+++ b/ext/net/ops_tls.rs
@@ -475,6 +475,8 @@ fn load_private_keys_from_file(
 pub struct ListenTlsArgs {
   alpn_protocols: Option<Vec<String>>,
   reuse_port: bool,
+  #[serde(default)]
+  load_balanced: bool,
 }
 
 #[op2]
@@ -502,7 +504,11 @@ where
     .next()
     .ok_or_else(|| generic_error("No resolved address found"))?;
 
-  let tcp_listener = TcpListener::bind_direct(bind_addr, args.reuse_port)?;
+  let tcp_listener = if args.load_balanced {
+    TcpListener::bind_load_balanced(bind_addr)
+  } else {
+    TcpListener::bind_direct(bind_addr, args.reuse_port)
+  }?;
   let local_addr = tcp_listener.local_addr()?;
   let alpn = args
     .alpn_protocols