[11u] RFR (S) 8260497: Shenandoah: Improve SATB flushing

[Roman Kennke]

Looks good, thank you!

Roman


> Original RFE:
>    https://bugs.openjdk.java.net/browse/JDK-8260497
>    https://git.openjdk.java.net/jdk/commit/316d52c1
> 
> There are distinct changes from master version.
> 
> First, SATBMarkQueue and PtrQueue are still closely coupled together. 
> Because of this, we cannot drop "virtual" from should_enqueue_buffer, 
> and therefore cannot remove 
> ShenandoahSATBMarkQueue::should_enqueue_buffer completely.
> 
> Second, there is no SATBMarkQueueSet::flush_queue(SATBMarkQueue&), and 
> so we have to go to SATBMarkQueue::flush directly. This also simplifies 
> ShenandoahFlushSATBHandshakeClosure.
> 
> I have observed reduced final-mark times with this patch.
> 
> 11u variant:
>    https://cr.openjdk.java.net/~shade/8260497/webrev.11u.01/
> 
> Testing: hotspot_gc_shenandoah; tier{1,2} with Shenandoah
> 
>