RFR: Fix ShenandoahStrDedupStress test OOM

[Zhengyu Gu]

Nightly tests showed ShenandoahStrDedupStress test OOM on static heuristics.

The solution is to reduce live data size, but increase string mutations 
to archive enough GC cycles. It increases test time slightly.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/strdedup_test_oom/webrev.00/

Test:
   jenkins test on gotland (fastdebug and release)
   manual test on polwarth (fastdebug and release)


Thanks,

-Zhengyu