CASSANDRA-9472 Reintroduce off heap memtables - patch to 3.0

[Andrew Whang]

I'm wondering if anyone has been able to patch CASSANDRA-9472 to 3.0,
without breaking unit tests. The patch was introduced in 3.4, but 3.0.x
contains unit tests and code from later 3.x releases, which makes debugging
unit test failures difficult - i.e. SSTableCorruptionDetectionTest, which
was introduced in 3.7 and is found in 3.0.14, but not in 3.4.