After CASSANDRA-8893, a RAR will be a very lightweight object and will not need caching, so we can eliminate this cache entirely. Instead we should have a pool of buffers that are page-aligned.
RandomAccessReader should share its FileChannel with all instances (via SegmentedFile)
Our default buffer size for (uncompressed) buffered reads should be smaller, and based on the expected record size
Allow zero-copy reads again
Improvements to BufferPool