Removing blocking code async HTTP request

The HTTP Client has a state pool. It blocks when we've exceeded max_concurrency.
This can block processing forever. A simple way to reproduce this is to go into
the demo cdp.js, and execute the XHR request 5 times (loading json/product.json)

To some degree, I think this is a result of weird / non-intuitive execution
flow. If you exec a JS with 100 XHR requests, it'll call our XHR _send function
but none of these will execute until the loop is run (after the script is done
being executed). This can result in poor utilization of our connection and
state pool.

For an async request, getting the *Request object is itself now asynchronous.
If no state is available, we use the Loop's timeout (at 20ms) to keep checking
for an available state.

src/app.zig

@@ -52,7 +52,8 @@ pub const App = struct {
             .telemetry = undefined,
             .app_dir_path = app_dir_path,
             .notification = notification,
-            .http_client = try HttpClient.init(allocator, 5, .{
+            .http_client = try HttpClient.init(allocator, .{
+                .max_concurrent = 3,
                 .http_proxy = config.http_proxy,
                 .tls_verify_host = config.tls_verify_host,
             }),