RFR (S): Update statistics to capture update-refs, partial root scans

[Aleksey Shipilev]

Hi,

This change adds the details on rootset scanning for update-refs and partial. It
also renames "Drain SATB" to "Finish Queues", which conveys the intent better.

Fix:
 http://cr.openjdk.java.net/~shade/shenandoah/stats-update-refs/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey