[Enhancement] Adding the ability to apply or discard a subset of keys

We have a fair complex form that we back with the buffered proxy and found that we could "stage" a bunch of changes inside of the buffer and then when the user submits the form apply only the subset that should be persisted instead of the whole buffer.

```js
actions: {
  submit: function() {
    this.applyBufferedChanges(this.get('keysToApply'));
    ...
  }
}
...

keysToApply: function() {
  // logic to determine which keys to persisted
}.property()
```
why not just.

```js
onlyTheseKeys.forEach(function(key) {
  delete this.buffer[key];
}, this);
```