RFR: Retry to fix ShenandoahStrDedupStree OOM

[Zhengyu Gu]

Old default live string size (3000000) is prone to OOM.

Reduce default live string size and increase string mutations for 
heuristics other than adapative, who seems to handle live size better 
and need bigger size to trigger enough GC cycles.

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

Test:

   3 iterations of tier3_gc_shenandoah on gotland (fastdebug and release)


Thanks,

-Zhengyu