MAX_DIRECT_MEM not respected by C*

[jhohertz]

Priam attempts to set the "-XX:MaxDirectMemorySize" setting for the JVM indirectly via an environment variable, MAX_DIRECT_MEM, which looks like it is supposed to be picked up by a custom conf/cassandra-env.sh script.

We're using the vanilla one from the C* distribution, which doesn't try to detect/set this as Priam is expecting.

Effect is that direct memory use will be unbounded until it is corrected.