RFR: Rework GC degradation on allocation failure

[Aleksey Shipilev]

On 05/17/2018 09:46 PM, Zhengyu Gu wrote:
> Hi,
> 
> ShenandoahMetricsSnapshot
> 
> - member variables need "_" prefix
> 
> ShenandoahMetrics::external_fragmentation()
> 
>  - algorithm does not seem to match comment
> 
> Otherwise, looks good.

Right. Does this look better?
  http://cr.openjdk.java.net/~shade/shenandoah/alloc-failure-rework/webrev.02/

I also reconsidered ShenandoahHeap::allocate_memory, and that fixes the issues that made nightly
tests fail.

Testing: hotspot_gc_shenandoah

-Aleksey