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

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.