Use SecureRandom.generateSeed() to generate ThreadLocalRandom's initialSeedUniquifier

Motivation:

Previously, we used SecureRandom.nextLong() to generate the initialSeedUniquifier.  This required more entrophy than necessary because it has to 1) generate the seed of SecureRandom first and then 2) generate a random long integer.  Instead, we can use generateSeed() to skip the step (2)

Modifications:

Use generateSeed() instead of nextLong()

Result:

ThreadLocalRandom requires less amount of entrphy to start up