api: Add new ListenBucketNotificationHandler. (#2336)

This API is precursor before implementing `minio lambda` and `mc` continous replication.

This new api is an extention to BucketNofication APIs.

// Request
```
GET /bucket?notificationARN=arn:minio:lambda:us-east-1:10:minio HTTP/1.1
...
...
```

// Response
```

{"Records": ...}
...
...
...
{"Records": ...}
```

routers.go

@@ -66,6 +66,10 @@ func configureServerHandler(srvCmdConfig serverCmdConfig) http.Handler {
 		ObjectAPI: objAPI,
 	}
 
+	// Initialize a new event notifier.
+	err = initEventNotifier(objAPI)
+	fatalIf(err, "Unable to initialize event notification queue")
+
 	// Initialize router.
 	mux := router.NewRouter()