Setting RNG in StratifiedCrossValidationSplitting for thread safety

Using the same CRandom instance over multiple threads is not safe.  Modified StratifiedCrossValidationSplitting so that we can set own RNGs using member m_rng.  This patch works for me when using different instances of CStratifiedCrossValidationSplitting across multiple threads.

@karlnapf: I think there's need for discussion.  Please have a look.