make use CachedThreadPool configurable.

By performance test, we found that the streaming performance decreased by more than 20% after RATIS-1419 was introduced into CachedThreadPool.
So we added a configuration that lets users choose whether to use CachedThreadPool or not.

Here are the results of a performance test using CachedThreadPool(Other configurations keep unchanged):

Here are the results of a performance test using newFixedThreadPool(Other configurations keep unchanged):