Jvm inplace predict

[trivialfis]

@yoquinjo Could you please take a look? This is a draft of the changes needed to use the internal inplace predict function based on your existing patch.

• Use the PredictFromDense function to implement inplace prediction, which doesn't make a data copy.

[trivialfis]

I haven't tested whether the JVM environment is thread-safe. The previous attempt with DMatrix got a race somewhere in the JVM data iterator with spark.

I'm not an expert in Java, the ideal case for prediction to return is to have an array/tensor with proper shape, which can be calculated from the C function. But since the Java land doesn't have a de-facto array implementation, a plain array is returned instead.

I might not be able to finish the full PR, feel free to merge if you find it useful.