[FEA][JNI] ContiguousTable.getBuffer should increase ref count of buffer

[abellina]

We currently have a class in JNI called ContiguousTable which tracks both a contiguous device memory buffer and metadata needed to recreate a cuDF table from it. These are the results of contiguous_split.

The problem is that right now we have code in the plugin that calls getBuffer on this object and then has to remember to increase ref counts, or otherwise be really careful about handling this buffer since it goes away once the ContiguousTable closes.

It would be nice to fix this so we can clean up code in the plugin.