Merge pull request #43 from changbl/working

Code improvement
Code fix

Delete DPID_TO_CONNS, DPID_TO_IP, GATEWAY;
Add transactions to RPC;
Delete unused "in_port" in _handle_arp_request;
Delete argument "migration" in update_position;
Merge pull request #44 from powerist/working

Code fix
Flow table size reduction

We reduce the table size by creating virtual MAC address.
Each guest is assigned a virtual MAC address,
created by the controller upon connection.
The format of virtual MAC:
dcenter:switch:vm_id:00

Forwarding is based on prefix matching, like that of
IP routing.
Merge pull request #45 from powerist/working

Flow table size reduction
Install switch-to-switch, datacenter-to-datacenter flow
on switch connection.
Merge pull request #46 from powerist/working

Install switch-to-switch, datacenter-to-datacenter flow on switch connection
New live migration

Preinstall all switch-level and datacenter-level flows;
Live migration involves two steps:
1. Add flow at dpid_new towards local guest;
2. Redirect flow at dpid_old towards new position.
Merge pull request #47 from powerist/working

New live migration
Add function set_flow
Merge pull request #48 from powerist/working

Add function set_flow
Fix DHCP bug due to variable name change