Improve property change notifications for records

[wecc]

This PR improves how we trigger property changes when pushing data to the store/a record.

Previously we only cared about comparing new data with model._data, this resulted in weird behavior in a couple of scenarios:

1. If you modify an attribute locally and push new canonical data that matched the current local attribute we got a false trigger.
2. If you modify an attribute locally and save, and the server return new canonical data that matches the existing canonical data, we did not get a trigger (hence the old locally changed value stayed).

Fixes #2937, fixes #2900

[igorT]

not sure this is correct, as if you consider user applying changes after sending to the server, even if server sends a different value, it will not show up once you get it as an attr, as the local change will win

[wecc]

if we're in the middle of a save we have values in �_inFlightAttributes and that will win over _attributes�.

var person = store.push('person', { id: 1, name: 'Tomster' });
person.set('name', 'Yehuda');
person.save();
// => request { name: 'Yehuda' }
person.set('name', 'Tom');
// <= response { name: 'Yehuda' }

_data will contain { name: 'Tomster' }
_attributes will contain { name: 'Tom' }
_inFlightAttributes will contain { name: 'Yehuda' }

We will compare payload ({ name: 'Yehuda' }) to the merged original ({ name: 'Yehuda' }) and trigger no change.

Isn't this the correct behavior? We have { name: 'Tom' } locally (since we changed that) and we still do, without triggering observer. If we want to get back to { name: 'Yehuda' } we should do a rollback.

[wecc]

However, if the server were to return something completely different, say { name: 'Igor' }, we would get a trigger, but the value would remain { name: 'Tom' }. That might be an issue.

[igorT]

Yes, but we would get even if you didn't merge in attributes, no?

[wecc]

This is required for the scenario in test https://github.com/emberjs/data/pull/2998/files#diff-56dbbdcd2b2c38cfe44c52bec2b99646R55

[igorT]

Hm, then the order seems wrong, it should be inflight then attributes?

[fivetanley]

Ember.create(null) instead of {}

[bmac]

@wecc what needs to happen for this to get merged?

[wecc]

@bmac needs to address the comments in here to make sure it's working as intended.

[igorT]

@bmac didn't you move this into a new pr?

[wecc]

@igorT it's in #3216