lib: Add Aggregate Table and Aggregate_node

lib: Add Aggregate Table and Aggregate_node

Add a abstraction for `struct route_node` and `struct route_table`
such that we can have an aggregate route_node and table.  This
is because only bgp/rfapi and ripng use the aggregate data pointer
in `struct route_node`.  For full route tables other routing
protocols and tables are paying a 8 byte overhead per node.
A full bgp table ends up being ~1.2 million routes in bgp
and zebra.  This is not an insiginificant amount of data.

So create the data structures for this replacement, but
do not replace the aggregate pointer yet.  This is because
later commits will convert rfapi and ripng over to this
new data, and finally we'll move the aggregate pointer.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>