RFR: Record cycle start/end in Traversal GC to avoid continuous periodic GC

[Aleksey Shipilev]

On 02/02/2018 12:57 AM, Roman Kennke wrote:
> Turns out Traversal is running into continuous back-to-back (periodic) GC
> because we're never actually recording cycle start/end :-) This patch fixes
> it. Gives a good boost to apps running with Traversal GC. Easily passes by
> adaptive heuristic now.
> 
> http://cr.openjdk.java.net/~rkennke/traversal-record-cycles/webrev.00/

Looks good.

We can (and probably should) encapsulate that in ShenandoahGCSession mark object that all cycles
seem to share well.

-Aleksey