Make "async" the default server model

[thekid]

Currently, we have the following server models:

• serve (the default): A single-threaded web server, blocks until one client's HTTP request handler has finished executing before serving the next request.
• async: Same as above, but handlers can yield control back to the server to serve other clients during lengthy operations such as file up- and downloads.
• prefork: Much like Apache, forks a given number of children to handle HTTP requests. Requires the pcntl extension.
• develop: As mentioned above, built ontop of the PHP development wenserver. Application code is recompiled and application setup performed from scratch on every request, errors and debug output are handled by the development console.

This issue suggests making async the new default for the next major release - version 3.0.0 at the time of writing.

[thekid]

First of all, I've created a branch named two so we can easily create more 2.X releases should they be necessary.

[thekid]

The basic implementation is easy:

diff --git a/src/main/php/xp/web/Runner.class.php b/src/main/php/xp/web/Runner.class.php
index 123c6de..75e732a 100755
--- a/src/main/php/xp/web/Runner.class.php
+++ b/src/main/php/xp/web/Runner.class.php
@@ -83,7 +83,7 @@ class Runner {
     $docroot= new Path($webroot, 'static');
     $address= 'localhost:8080';
     $profile= getenv('SERVER_PROFILE') ?: 'dev';
-    $mode= 'serve';
+    $mode= 'async';
     $arguments= [];
     $config= [];
     $source= '.';

However, we should rename the serve mode to single or blocking, e.g.

[thekid]

However, we should rename the serve mode to single or blocking, e.g.

sequential

"following one after another without others coming in between"