Bug 1039465 - [Contacts DataRefactor] Tweak global_contacts_data module ...

...and multi_contact
This could be made generic for any number of ids:

```javascript
var getEntry = function getEntry(globalDsId) {
  return getDatastore().then(function success(store) {
    return store.get.apply(store, arguments);
  });
};
```

So that `getData` and `getContactData` could all benefit (I modified them below).