Size of in-memory compaction thread pool should be configurable

  private static final int POOL_SIZE = 10;
  private static final ThreadPoolExecutor INMEMORY_COMPACTION_POOL =
      new ThreadPoolExecutor(POOL_SIZE, POOL_SIZE, 60, TimeUnit.SECONDS,
          new LinkedBlockingQueue<>(),
          new ThreadFactory() {
            @Override
            public Thread newThread(Runnable r) {
              String name = Thread.currentThread().getName() + "-inmemoryCompactions-" +
                  System.currentTimeMillis();
              return new Thread(r, name);
            }
          });