Optimation for _update_lrouter_routes()

[Dong Jun]

Combine multi routers routes update in _subnet_update into a single transaction.

def _subnet_update(resource, event, trigger, **kwargs):
    ...
    for router_id in router_ids:
        l3plugin._update_lrouter_routes(context, router_id, add, remove)

[Lucas Alvares Gomes (lucasagomes)]

Fixed at https://review.opendev.org/#/c/525996/