RFR: Refactor alive-closures to deal better with new marking contexts

[Aleksey Shipilev]

On 07/19/2018 12:43 AM, Roman Kennke wrote:
> Turns out this was easier to do than I suspected. RefProc does already
> take NULL alive-closure and resets it when used. This means that we
> already carried unnecessary cruft in the form of our static
> alive-closure and forwarded-alive-closure. The tricky part was to
> abstract out the selection logic, this is now in a little helper class
> called ShenandoahIsAlive.
> 
> http://cr.openjdk.java.net/~rkennke/aliveclosures/webrev.00/
> 
> Testing: tier1_gc_shenandoah passed, pending tier3_gc_shenandoah before push

Looks okay. I would rename ShenandoahIsAlive -> ShenandoahIsAliveSelector

-Aleksey