[R-package] Fix best_iter and best_score (#2159)

* Callback for NA handling

* lgb.Booster default score => NA

* lgb.cv default best score => NA

* Fix back callback

* lgb.train with booster check at the end

manual tests done: 
* With early stopping + with validation set
* With early stopping + without validation set
* Without early stopping + with validation set
* Without early stopping + without validation set

And with multiple metrics / validation sets.

* lgb.cv with booster check at the end

manual tests done: 
* With early stopping + with validation set
* With early stopping + without validation set
* Without early stopping + with validation set
* Without early stopping + without validation set

And with multiple metrics / validation sets.