Zookeeper failover:

Handle failover for packet_in event.
Whenever a packet_in event is received, do logging first:
create a path on Zookeeper: /failover/"$dpid,$in_port",
whose value is msg.data, namely the raw ethernet packet.

During the process of packet_in event, all write operations
to Zookeeper, are bundled as one transaction, which is committed
after all other operations are finished.