RFR: Fix and enable suspendible workers by default

[Roman Kennke]

Look at this webrev instead. I remove stray includes in 2 non-Shenandoah
files that slipped in by accident:


http://cr.openjdk.java.net/~rkennke/fix-enable-susp-workers/webrev.01/

I'm also currently running benchmarks to check that it doesn't possibly
regress performance.

Thanks,
Roman

> http://cr.openjdk.java.net/~rkennke/fix-enable-susp-workers/webrev.00/
> 
> Testing: tier3_gc_shenandoah ok
> (ShenandoahStrDedupStress times out, but that was pre-existing and
> doesn't seem related. It appears that we should cut down the test work
> there, or make it time-based to begin with.)
> 
> I'll post a jdk8u patch shortly. This requires much more work, because
> the stuff is not even existent in jdk8u, and STS has slightly different
> APIs too.
> 
> Roman
>