Use cached thread pools in DataStreamManagement

In DataStreamManagement, the requestExecutor and writeExecutor are created by Executors.newFixedThreadPool(..). A drawback is that the threads in the fixed thread pool are using resource even if the executors are idle.

One way to improve it is to use Executors.newCachedThreadPool(..). However, all the newCachedThreadPool(..) methods provided by Executors do not take a maximumPoolSize parameter. We will define a newCachedThreadPool(..) method which takes a maximumPoolSize parameter.