Fix reference count on Java DeviceMemoryBuffer after contiguousSplit [skip ci]

[jlowe]

After #5815 columns created from a contiguous table are not incrementing the refcount of the underlying device memory buffer. That means columns are no longer being "sliced" from the buffer such that they are able to hold the buffer open after the buffer closes. The RAPIDS Accelerator for Apache Spark relies upon this behavior.

This updates the ColumnVector constructor for a column built from a contiguous table to increment the reference count of the underlying buffer and add it to the list of items to close when the column is finally closed. To avoid having to explicitly slice the underlying buffer and create unnecessary object overhead, I exposed the reference counting as a method as has been done in other cudf Java classes.

[GPUtester]

Please update the changelog in order to start CI tests.

View the gpuCI docs here.

[kuhushukla]

Thanks for catching this Jason. Fix looks good to me.