Check if data is identical before telling :Save() its changed

The following flow:
```
- Get() the data store, the value is { foo = "bar" }
- Set({ foo = "bar" })
- Save()
```

...results in a superfluous save.

Instead, Save() should check the previous data to see if it is identical.

Start by locating the data-store implementation and its Get(), Set(), and Save() entry points, then reproduce the flow described in the issue. Done means Save() recognizes identical data and avoids a superfluous save while still saving changed data.

Written by the indexing model from the issue text.