RFR: [Cherry pick] 8204947: Port ShenandoahTaskTerminator to mainline and make it default

[zgu at redhat.com]

I would like to cherry pick upstream changeset for 8204947 to
Shenandoah repo.


# HG changeset patch
# User zgu
# Date 1544208906 18000
#      Fri Dec 07 13:55:06 2018 -0500
# Node ID bec57b4a6d69651d68087efc2b9b66c6f7827c98
# Parent  d2f118d3f8e7ca6cfb92808fa7fabe67b3e4b03f
8204947: Port ShenandoahTaskTerminator to mainline and make it default
Reviewed-by: tschatzl, rkennke

Thanks,

-Zhengyu