[xgb] Fix xgb intern to close replaced array

[ewan0x79]

Description

close replaced array for xgb engine

[frankfliu]

This is not necessary, the instanceof will check null:

if (!(replaced instanceof XgbNDArray))

[frankfliu]

NDArray operation is not thread-safe. Adding synchronized here won't improve thread-safty

[frankfliu]

Not sure about this null check, if we want to prevent a closed NDArray, we should just do:

if (this.closed()

[frankfliu]

The memory leak issue may caused by alternativeArray leak, I think we should close current array's alternative array as will. The code can be re-implemented as the following:

        if (!(replaced instanceof XgbNDArray)) {
            throw new IllegalArgumentException(
                    "The replaced NDArray must be an instance of XgbNDArray.");
        }
        XgbNDArray array = (XgbNDArray) replaced;
        if (isReleased()) {
            throw new IllegalArgumentException("This array is already closed");
        }
        if (replaced.isReleased()) {
            throw new IllegalArgumentException("This target array is already closed");
        }

        long pointer = handle.getAndSet(0L);
        JniUtils.deleteDMatrix(pointer);
        if (alternativeArray != null) {
            alternativeArray.close();
        }

        data = array.data;
        handle = array.handle;
        format = array.format;
        alternativeArray = array.alternativeArray;
        array.handle = null;
        array.alternativeArray = null;
        array.close();