TCP auto-merge support in NSR for BGP and LDP

This feature enables TCP auto-merge in nonstop active routing (NSR) for protocols (BGP, LDP) (kernel). NSR auto-merge is one of the kernel components of the socket replication. On switchover, this component merges the socket pairs automatically from the secondary to the primary Routing Engine. 

Currently, NSR switchover from secondary to primary happens when rpd issues a merge call for each secondary socket pair to merge them to a single socket, which could result in a delay. To avoid this delay, this feature introduces an auto-merge module in the kernel that decouples the secondary socket merge from rpd and automatically merges secondary sockets on switchover so that the rpd high priority thread takes advantage of this and generates faster keep-alive to sustain TCP connections on switchover.