RFR: Make Shenandoah to use ShenandoahOWST only

[Zhengyu Gu]

This patch to remove UseShenandoahOWST experimental flag and have 
Shenandoah to use OWST termination protocol only.

ShenandoahOWST has been enabled shortly after it was introduced, it is 
well tested, let's eliminate the flag and purge fallback branch. This 
also reverts some of upstream changes.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/sh-owst/webrev.00/index.html


Test:

   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu