LightGBM

fix address alignment, required by cran (#3415)

* fix dataset binary file alignment

* many fixes

* fix warnings

* fix bug

* Update file_io.cpp

* Update file_io.cpp

* simplify code

* Apply suggestions from code review

* general

* remove unneeded alignment

* Update file_io.h

* int32 to byte8 alignment

* Apply suggestions from code review

* Apply suggestions from code review