RFR: Fix single-threaded reference processing

[Aleksey Shipilev]

On 07/20/2018 02:30 PM, Roman Kennke wrote:
> With the new worker-scaling, we might end up running ref-proc
> single-threaded, in which case our reserved queues and num-workers don't
> match, and as Zhengyu already noted, we are not in OOM scope in
> traversal GC. The single-threaded ref-proc doesn't shoot up any workers,
> but runs in VMThread instead.
> 
> The fix is to also push/pop the reserved queues correctly: first,
> prepare for single-threaded (in case we don't go through the executor),
> then have the executor override/restore it in case we go there.
> 
> The fix also contains Zhengyu's fix for traversal GC. The OOM scope is
> not needed for conc-mark.
> 
> http://cr.openjdk.java.net/~rkennke/singlethreadrefproc/webrev.00/

OK, fine.

-Aleksey