RFR (XS): 8201487: Do not rebalance reference processing queues if not doing parallel reference processing

[Kim Barrett]

> On Apr 12, 2018, at 4:03 PM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> 
> Hi all,
> 
>  can I have reviews for this small change that removes discovered list
> rebalancing for better work distribution across threads if running
> serially.
> 
> This can save hundreds of ms in reference processing time.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8201487
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8201487/webrev/
> Testing:
> hs-tier1-3
> 
> Thanks,
>  Thomas

Looks good.