bgpd: Tie in more clear events to clear code

bgpd: Tie in more clear events to clear code

The `clear bgp *` and the interface down events
cause a global clearing of data from the bgp rib.
Let's tie those into the clear peer code such
that we can take advantage of the reduced load
in these cases too.

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