bgpd: Code to remove the bottleneck in aggregation.

bgpd: Code to remove the bottleneck in aggregation.

The code that causes the bottleneck has been written generically to
handle the below two cases:
a) When a new aggregate-address is configured.
b) When new routes, that can be aggregated under an existing
aggregate-address, are received.
This change optimizes the code that handles case-(b).

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