RFR: Make Shenandoah string deduplication CDS aware

[Zhengyu Gu]

Following g1's string deduplication, we should also populate archived 
string to string deduplication table.

Webrev: http://cr.openjdk.java.net/~zgu/shenandoah/dedup-cds/webrev.00/

Test:
   hotspot_gc_shenandoah + new test on Linux x64 (release + fastdebug)


Thanks,

-Zhengyu