RFR: Eliminate string dedup cleanup phase and correct UR closure

[Zhengyu Gu]

This patch addresses two issues:

1) We don't need cleanup phase. It was intended to cleanup dead objects 
in string dedup table, so that we won't deduplicate to those *dead* char 
arrays.

However, deduplication applies keep_alive_barrier() to revive those 
objects, that makes this phase unnecessary.

2) Applied wrong UR closure.
We need to update references for all marked objects in completed bitmap, 
but ShenandoahUpdateRefsClosure only updates references in current cset.

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

Test:
   hotspot_gc_shenandoah (fastdebug + release)


Thanks,

-Zhengyu