bgpd: Convert from struct bgp_node to struct bgp_dest

bgpd: Convert from struct bgp_node to struct bgp_dest

This is based on @donaldsharp's work

The current code base is the struct bgp_node data structure.
The problem with this is that it creates a bunch of
extra data per route_node.
The table structure generates ‘holder’ nodes
that are never going to receive bgp routes,
and now the memory of those nodes is allocated
as if they are a full bgp_node.

After splitting up the bgp_node into bgp_dest and route_node,
the memory of ‘holder’ node which does not have any bgp data
will be allocated as the route_node, not the bgp_node,
and the memory usage is reduced.
The memory usage of BGP node will be reduced from 200B to 96B.
The total memory usage optimization of this part is ~16.00%.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>
Signed-off-by: Yuqing Zhao <xiaopanghu99@163.com>