[4.2] Select From Write Pdo

[taoso]

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:

$user = User::onWrite()->find(1);

[GrahamCampbell]

Please put braces on their own lines.

[GrahamCampbell]

Please fix the inline issues, then squash to one commit.

[taoso]

@GrahamCampbell sorry for troubles. And coding style has been adjusted at eb9d915

[lucasmichot]

Need docblock