RFR (S) 8233520: Shenandoah: do not sleep when thread is attaching

[Roman Kennke]

Ok.

Thanks,
Roman


> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8233520
> 
> Fix:
>   http://cr.openjdk.java.net/~shade/8233520/webrev.01/
> 
> This was exposed by recently added assert. But the bug itself is legit Shenandoah bug, and should be
> fixed everywhere.
> 
> Testing: affected test; hotspot_gc_shenandoah
>