ref: 4db9c329762779189ece5ff46bec402c62792eff
parent: d4f09d9048f5d7388358d3838fe02c5377cb79b9
author: sijchen <sijchen@cisco.com>
date: Wed Mar 2 12:08:09 EST 2016
remove sink in WelsThreadPool and hide the construtor to finish the singleTon