totemnet: Add totemnet_member_set_active

totemnet_member_set_active together with transport specific
member_set_active makes possible for totemnet (and more interestingly
transport) to be informed about membership changes.

Signed-off-by: Jan Friesse <jfriesse@redhat.com>
Reviewed-by: Christine Caulfield <ccaulfie@redhat.com>