NSR for OSPFv2

This feature introduces nonstop active routing for OSPFv2. Nonstop active routing provides high availability for Routing Engines by enabling transparent switchover of the Routing Engines without requiring restart of supported routing protocols. Both Routing Engines are fully active in processing protocol sessions, and so each can take over for the other. The switchover is transparent to neighbor routing devices, which do not detect that a change has occurred.