[4.2] Database: Select from write pdo

Eloquent use the read pdo for select query. 

However, in some scenes, it needs to query data just from the write pdo(master db), because of the *delay* of database syncing.

As @crynobone says(#6599), the demand can be satisfied by the use of `User::on('mysql::write')->find(1);`. But, in my opinion, using the `on` method is not a natural way. Because every Eloquent model knows which db connection they will use. There should be no need to use the `on` method to set a concrete connection name.

Here is my solution. I add a new `onWrite` method to the Eloquent. We could do:

```php
$user = User::onWrite()->find(1);
```
Please put braces on their own lines.