RFR: Restore string table rehashing task in cleanup tasks

[Zhengyu Gu]

I wrongly removed string table rehashing task from safeppont cleanup 
phase, thought it should be performed concurrently.

I believe that StringInternCleanup test failure is due to upstream bug - 
I filed JDK-8205006 (https://bugs.openjdk.java.net/browse/JDK-8205006) 
and temporary disable the test till the bug is fixed.


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

Test:

   tier3_gc_shenandoah.

Thanks,

-Zhengyu