The current implementation doesn't work with destroyAll()

[mittinatten]

Hi,
Thanks for concise and very useful example! I added this functionality to my project very easily based on your description.

I ran into some trouble in my tests where I call PersistedModel.destroyAll() a lot. In that call, ctx.where.id is not defined. Something like this should work for both destroyById() and destroyAll() (adapted from a project where I didn't use promises, and handle errors differently):

Cat.observe('before delete', function(ctx, next) {
  Cat.find(ctx.where, (err, cats) => {
    if (err) return next(err);

    if (cats.length === 0) return next();

    let deletions = cats.map(cat => {
      return {
        'delete': {
          _index: 'cat',
          _type: 'cat',
          _id: cat.id,
        }
      }
    });
    client.bulk({body: deletions}, next);
  });
});