BGP: Do route refresh after preference change

BGP: Do route refresh after preference change

Reconfiguration of preference is handled by nest code by asking for
reload, but in case of BGP with import table, that just reloaded routes
with the old preference. In BGP, we can handle that by triggering full
route refresh.

Although, it would be probably better to set preference in nest, when
a route is propagated from the import table.