BGP: Add dynamic update group support

BGP: Add dynamic update group support

This patch implements the 'update-groups' functionality in BGP. This is a
function that can significantly improve BGP performance for Update generation
and resultant network convergence. BGP Updates are formed for "groups" of
peers and then replicated and sent out to each peer rather than being formed
for each peer. Thus major BGP operations related to outbound policy
application, adj-out maintenance and actual Update packet formation
are optimized.

BGP update-groups dynamically groups peers together based on configuration
as well as run-time criteria. Thus, it is more flexible than update-formation
based on peer-groups, which relies on operator configuration.

[Note that peer-group based update formation has been introduced into BGP by
Cumulus but is currently intended only for specific releases.]

From 11098af65b2b8f9535484703e7f40330a71cbae4 Mon Sep 17 00:00:00 2001
Subject: [PATCH] updgrp commits