[R-package] exclude training data from being checked for early stopping

[StrikerRUS]

Check all datasets in validation list for early stopping excluding the training data to match the Python-package behavior:

Early stopping requires at least one set in valid_sets. If there is more than one, it will use all of them except the training data:
https://github.com/microsoft/LightGBM/blob/master/docs/Python-Intro.rst#early-stopping

Refer to #2371.
Python code:
https://github.com/microsoft/LightGBM/blob/5c0daca0a1ae601aa2bdcaa0d2fc16bcf3be150a/python-package/lightgbm/callback.py#L238-L241

C++ code:
https://github.com/microsoft/LightGBM/blob/5c0daca0a1ae601aa2bdcaa0d2fc16bcf3be150a/src/io/config.cpp#L237-L248

[StrikerRUS]

Closed in favor of being in #2302. We decided to keep all feature requests in one place.

Welcome to contribute this feature! Please re-open this issue (or post a comment if you are not a topic starter) if you are actively working on implementing this feature.

[jameslamb]

this was fixed in #2961