FIB scaling and performance enhancements

The Packet Forwarding Engine is enhanced to scale and support a higher number of routes in the forwarding information base (FIB), also known as forwarding table. However, during graceful Routing Engine switchover (GRES), when there are ten million IPv4 routes in the forwarding table, there is traffic loss. This traffic loss is not seen when a routing protocol process (rpd) runs in warm standby mode. We currently do not support unified ISSU and NSR at this scale.