memtable management in Cassandra

Moving data structures off of the Java heap to native memory, keeps up the datasets to continue to grow.

offheap_buffers moves the cell name and value to DirectBuffer objects. This has the lowest impact on reads -- the values are still "live" Java buffers -- but only reduces heap significantly when you are storing large strings or blobs.

offheap_objects moves the entire cell off heap, leaving only the NativeCell reference containing a pointer to the native (off-heap) data. This makes it effective for small values like ints or uuids as well, at the cost of having to copy it back on-heap temporarily when reading from it.

Tune the memtable_offheap_space_in_mb to reasonable value for your system.

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