[7.x] Add usingConnection method to DatabaseManager

[antonkomarev]

This PR adds usingConnection method to Illuminate\Database\DatabaseManager.

It is useful when you want to execute some actions in one connection and then switch back to the original connection.

// Some logic performed on tenant connection

$this->databaseManager->usingConnection('landlord', function () {
    // Some logic performed on landlord connection
});

// Some logic performed on tenant connection