Issue 51072 - Set the default minimum worker threads
    
    Description:  Testing has shown that using current number of CPU cores
                  to set the thread number gives the best performance, but
                  when there are expensive operations total throughput drops.
    
                  We still need a minimum number of workers threads to handle
                  a wide range of operations.  We decided for now that the
                  minimum should be 16 workers.
    
    relates: https://pagure.io/389-ds-base/issue/51072
    
    Reviewed by: tbordaz & firstyear (Thanks!!)
    
    Define MAX and MIN threads, and improve logging