RFR: Make Shenandoah to use ShenandoahOWST only

[Roman Kennke]

Good to me.

Is there any chance to also get rid of the other upstream diffs? Or are
those needed for the OWST termination protocol stuff?

Thanks,
Roman

> 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