RFR: Revert Shenandoah changes in taskqueue.hpp

[zgu at redhat.com]

Let's revert the changes in taskqueue.hpp, so we can apply upstream TQ
patch cleanly.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/revert_tq/webrev.00/

Thanks,

-Zhengyu