Multi DB Batch writes

Hi all,

Im currently implementing regional db's in my organization and Im wondering if batchWrites will use the internal firebase ref to target the correct database or if all operations (set, update, delete, etc.) will use be written to the database the batch was created from.

e.g.
```js
...
dbp1 = admin.app('us').firestore().doc('/path/to/doc/us');
dbp2 = admin.app('ca').firestore().doc('diff/path');

const batch = admin.app('ca').firestore().batch();
batch.set(dbp1, { updatedAt: now });
batch.set(dbp2, { updatedAt: now });
batch.commit()
...
```
^^^ Will this write to both 'us' and 'ca' databases or just the 'ca' one.