[8] RFR: Shenandoah: add root statistics for string dedup table/queues

[Aleksey Shipilev]

This is sh/jdk8-specific fix to add root statistics for string dedup table/queues. This would match
what sh/jdk11 and jdk/jdk are doing.

Webrev:
 https://cr.openjdk.java.net/~shade/shenandoah/8u-strdedup-root-stats/webrev.01/

Testing: hotspot_gc_shenandoah, eyeballing gc logs with string dedup enabled

-- 
Thanks,
-Aleksey