net.http: add graceful shutdown .stop() method to the http.Server struct (#11233)

vlib/net/http/server_test.v

@@ -0,0 +1,16 @@
+module http
+
+import time
+
+fn test_server_stop() ? {
+	server := &Server{
+		accept_timeout: 1 * time.second
+	}
+	t := go server.listen_and_serve()
+	time.sleep(250 * time.millisecond)
+	mut watch := time.new_stopwatch()
+	server.stop()
+	assert watch.elapsed() < 100 * time.millisecond
+	t.wait() ?
+	assert watch.elapsed() < 999 * time.millisecond
+}