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
```

1 files changed, 4 insertions, 4 deletions

diff --git a/cli/worker.rs b/cli/worker.rs
index 82051da6c..7f04e8846 100644
--- a/cli/worker.rs
+++ b/cli/worker.rs
@@ -414,6 +414,7 @@ impl CliMainWorker {
   }
 }
 
+#[derive(Clone)]
 pub struct CliMainWorkerFactory {
   shared: Arc<SharedWorkerState>,
 }
@@ -546,7 +547,7 @@ impl CliMainWorkerFactory {
     let maybe_inspector_server = shared.maybe_inspector_server.clone();
 
     let create_web_worker_cb =
-      create_web_worker_callback(mode, shared.clone(), stdio.clone());
+      create_web_worker_callback(shared.clone(), stdio.clone());
 
     let maybe_storage_key = shared
       .storage_key_resolver
@@ -739,7 +740,6 @@ impl CliMainWorkerFactory {
 }
 
 fn create_web_worker_callback(
-  mode: WorkerExecutionMode,
   shared: Arc<SharedWorkerState>,
   stdio: deno_runtime::deno_io::Stdio,
 ) -> Arc<CreateWebWorkerCb> {
@@ -752,7 +752,7 @@ fn create_web_worker_callback(
         args.permissions.clone(),
       );
     let create_web_worker_cb =
-      create_web_worker_callback(mode, shared.clone(), stdio.clone());
+      create_web_worker_callback(shared.clone(), stdio.clone());
 
     let maybe_storage_key = shared
       .storage_key_resolver
@@ -802,7 +802,7 @@ fn create_web_worker_callback(
         disable_deprecated_api_warning: shared.disable_deprecated_api_warning,
         verbose_deprecated_api_warning: shared.verbose_deprecated_api_warning,
         future: shared.enable_future_features,
-        mode,
+        mode: WorkerExecutionMode::Worker,
         serve_port: shared.serve_port,
         serve_host: shared.serve_host.clone(),
       },