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": ...}
```

fs-v1.go

@@ -137,12 +137,15 @@ func newFSObjects(disk string) (ObjectLayer, error) {
 		shutdownFS(storage)
 	})
 
-	// Return successfully initialized object layer.
-	return fsObjects{
+	// Initialize fs objects.
+	fs := fsObjects{
 		storage:      storage,
 		physicalDisk: disk,
 		listPool:     newTreeWalkPool(globalLookupTimeout),
-	}, nil
+	}
+
+	// Return successfully initialized object layer.
+	return fs, nil
 }
 
 // StorageInfo - returns underlying storage statistics.