RFR: Use heuristics to determine the number of conc threads for each conc gc cycle

[Zhengyu Gu]

This is an experimental heuristics that determines the number of concurrent threads for each concurrent GC cycle.

SPECjbb runs do not show obvious improvement, it seems to ramp up load quickly, so conc thread count stays high.


The change set also contains some cleanup.


http://cr.openjdk.java.net/~zgu/shenandoah/conc-worker-heuristics/webrev.00/


Test:
   SPECjbb, some of SPECjvm benchmarks.


Thanks,

-Zhengyu