Some code optimizations

[ovalseven8]

• Make 'Cookie' class actually immutable (as described in comment)
• Use static members in 'RandomHelper' for efficiency and security reasons
• Synchronize method calls in 'CombinedSequence' class to make it thread safe (as described in comment)
• Use 'Collections.unmodifiableList' instead of int array because an int array is a mutable Java object
• Some other minor changes

[ovalseven8]

I accidentally deleted the branch. However, the important parts are still visible through the comments. Once they are debated, I can create a new PR with the final changes.