Read-only controllers

[changbl]

For performance: read-optimized, single-writer controllers

Upon start, all controllers read data from ZooKeeper data backend. During operation, we use a queue (e.g., RabbitMQ, AMQP) to buffer all the delta changes from the master/writer controller, and make all the read-only controller to deque from the queue, and update their cache in memory. Read-only controller speed up read performance, mainly ARP requests.