bgpd: Code to handle BGP aggregate's as-path.

bgpd: Code to handle BGP aggregate's as-path.

With this commit:
1) 'struct bgp_aggregate' is moved to bgp_route.h from bgp_route.c
2) Hashes to accommodate the as-path, communities, extended-communities and
   large-communities attributes of all the routes aggregated by an
   aggregate route is introduced in 'struct bgp_aggregate'.
3) Place-holders for the aggregate route's as-path, communities,
   extended-communities and large-communities attributes are introduced in
   'struct bgp_aggregate'.
4) The code to manage the as-path of the routes that are aggregatable under
   a configured aggregate-address is introduced.
5) The code to compute the aggregate-route's as-path is introduced.

Signed-off-by: NaveenThanikachalam <nthanikachal@vmware.com>