[dev] Agent RPC version does not auto upgrade if neutron-server restart first

[LIU Yulong]

For instance:
This neutron server was restarted 6 seconds earlier than l3-agent with a RPC version upgrading:
http://logs.openstack.org/71/633871/5/check/neutron-grenade-dvr-multinode/3fcd71c/logs/screen-q-svc.txt.gz#_Feb_10_06_32_10_279268
http://logs.openstack.org/71/633871/5/check/neutron-grenade-dvr-multinode/3fcd71c/logs/screen-q-l3.txt.gz#_Feb_10_06_32_16_750133

And then, we met many UnsupportedVersion exception:
http://logs.openstack.org/71/633871/5/check/neutron-grenade-dvr-multinode/3fcd71c/logs/screen-q-svc.txt.gz#_Feb_10_06_38_05_749257

This issue can also be seen in our local dev branch. A workaround is restart the agent first, and then the neutron server.