Remove Topology.dpid_to_dpid

Instead, use ip_to_dpid and dpid_ip_to_port to implement
get_fwd_port() and get_neighbors()
Merge pull request #101 from changbl/working

Remove Topology.dpid_to_dpid
Failover with zookeeper

This version has normal functionality
and basic failover without queue.

1. Failover for packet_in handling
2. Failover for RPC
3. Fix bugs introduced by previous commit