Y.Model.Vanilla on the server side instead of Y.Model

Y.Model and Y.Model.Base seems to be consirable slower than vanilla, which uses the same API but does not rely on Y.BaseCore. The preliminary results are:

```
new Y.Model.Vanilla() x 119,220 ops/sec ±1.03% (97 runs sampled)
new Y.Model.Base() x 23,223 ops/sec ±2.95% (92 runs sampled)
new Y.Model() x 14,673 ops/sec ±2.56% (79 runs sampled)
```

> Fastest is new Y.Model.Vanilla()

These results are based on this operation:

```
(new Y.Model.Vanilla({miami: 'florida'})).toJSON();
```

which is pretty much what we do internally per mojits with `mojito-data-addon` enabled.