datapath: Implement vxlan-soe

This patch implements vxlan-soe:
    http://tools.ietf.org/html/draft-zhou-li-vxlan-soe-01

Tested VXLAN throughput between two hypervisors, and the performance
gain of vxlan-soe is significant.
netperf TCP_STREAM test result:

Before the change: 2.62 Gbits/sec
After the change: 6.68 Gbits/sec
Speedup is ~250%.