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

...and multi_contact
I believe this function can be simplified to this:

```javascript
function getContactData(contactIds) {
  // store.get for the moment does not support array of ids but
  // a variable number of arguments. Probably bug 1038661 will fix that
  return store.get.apply(store, contactIds).then(function success(entries) {
    if (contactIds.length === 1) {
      entries = [entries];
    }
    var operations = entries.map(function(aEntry, i) {
      return MultiContact.getData({
        id: contactIds[i],
        entryData: aEntry
      });
    });
    return Promise.all(operations);
  });
}
```