cassandra: Enable -XX:ThreadPriorityPolicy=42

Cassandra startup script enables -XX:ThreadPriorityPolicy=42 and so
should we. It is disabled for historical reasons because when we first
started running Cassandra on OSv, we didn't support thread priorities at
all.

Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>