Application restart introduced.

When processing a restart request, the router sends a QUIT message to all
existing processes of the application.  Then, a new shared application port is
created to ensure that new requests won't be handled by the old processes of
the application.

test/python/restart/v2.py

@@ -0,0 +1,7 @@
+import os
+
+def application(environ, start_response):
+    body = "v2".encode()
+
+    start_response('200', [('Content-Length', str(len(body)))])
+    return [body]