Provide graceful-restart and long-lived graceful-restart support

In Release 3.2, support to Graceful Restart (GR) and Long Lived Graceful Restart (LLGR) helper modes to contrail-controller was provided. (https://github.com/Juniper/contrail-controller/wiki/Graceful-Restart)

In Release 4.0, this shall be extended to agents also. Agents would cache configuration and operational data in non-volatile memory and use the same to reconstruct its state after graceful restart. This helps in minimizing loss for existing traffic flows.

Agent spec
https://github.com/Juniper/contrail-controller/blob/master/specs/agent-resource-manager.md