RFR: Code roots scanning might be bypassed during degenerated cycle

[Aleksey Shipilev]

On 02/22/2018 07:28 PM, Zhengyu Gu wrote:
> Degenerated cycle might skip concurrent cycle and dive directly into final mark. When
> ShenandoahConcurrentScanCodeRoots is set, that means we may miss the scanning of code roots.
> 
> Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/code_roots/webrev.00/

Excellent!

Let me push my hs_err changes first.

Thanks,
-Aleksey