Attributes for concurrent_compactors and a few JVM GC tuning parameters

Please find PR which adds attributes for:

  * `concurrent_compactors` setting in `cassandra.yaml`
  * `-XX:SurvivorRatio=X` JVM parameter in `cassandra-env.sh`
  * `-XX:MaxTenuringThreshold=X`
  * `-XX:CMSInitiatingOccupancyFraction=X`

Quick [check against cassandra master](https://github.com/apache/cassandra/blob/f5866ca2bac8ce530bb4e20832ff61e622206815/conf/cassandra.yaml#L538) suggests that `concurrent_compactors` is still a valid parameter, so there's no need to put it behind `cassandra_old_version_20`.