Add request/reply event support to support communication between apps

What most of applications need to do is:

1) register a handler to catch a specific event
2) The handler does multiple asynchronous tasks (query dbs, configures
switches, etc).

2) can be implemented in an asynchronous way, that is sending multiple
requests at the same time. However, I don't think most of developers
need to do or want to do. The API to handle multiple asynchrnous tasks
in a synchronous way is more handy.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

ryu/base/app_manager.py

@@ -62,6 +62,7 @@ class RyuApp(object):
         self.observers = {}
         self.threads = []
         self.events = Queue()
+        self.replies = Queue()
         self.threads.append(gevent.spawn(self._event_loop))
 
     def register_handler(self, ev_cls, handler):
@@ -84,6 +85,15 @@ class RyuApp(object):
 
         return observers
 
+    def send_reply(self, rep):
+        SERVICE_BRICKS[rep.dst].replies.put(rep)
+
+    def send_request(self, req):
+        req.src = self.name
+        self.send_event(req.dst, req)
+        # going to sleep for the reply
+        return self.replies.get()
+
     def _event_loop(self):
         while True:
             ev = self.events.get()

ryu/controller/event.py

@@ -18,3 +18,15 @@
 class EventBase(object):
     # Nothing yet
     pass
+
+
+class EventRequestBase(EventBase):
+    def __init__(self, dst):
+        super(EventRequestBase, self).__init__()
+        self.dst = dst
+
+
+class EventReplyBase(EventBase):
+    def __init__(self, dst):
+        super(EventReplyBase, self).__init__()
+        self.dst = dst