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

[Thomas Schatzl]

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