CASSANDRA-15125 Replace Math.random with Replace.nextDouble to remo…

[bd2019us]

…ve performance overhead

CASSANDRA-15125 Math.random has a slight performance overhead associated with itself that does not exist in Random.nextDouble. These instances of Math.random, such as those sampled below, can be replaced with random.nextDouble. Furthermore, the randomness can be configured via the Random class if such a feature is needed in the future.