[FEA] Replace whole-buffer slicing with reference count increment

[jlowe]

There are a number of places in the plugin where we slice an entire memory buffer, i.e.: .slice(0, buffer.getLength()), to increment the reference count on the buffer. Every slice of a memory buffer creates two objects, the sliced buffer result and a sliced buffer cleaner.

This can add up when slicing during compressed shuffle, where each compressed buffer is whole-sliced by every column in every partition essentially just to increment the reference count. cudf recently added incRefCount to memory buffers, and it would create a lot less object garbage on the heap to call incRefCount on the buffer directly rather than slicing the whole buffer.