RFR: Separate Full GC root operations in GC stats

[Aleksey Shipilev]

The only thing that Full GC shares with Init Mark is root operations. Sometimes
what is clearly the Full GC root operation gets reported under Init Mark,
confusing the analysis.

Patch:
  http://cr.openjdk.java.net/~shade/shenandoah/stats-fullgc-roots/webrev.01/

Sample output:
  http://cr.openjdk.java.net/~shade/shenandoah/stats-fullgc-roots/perf.txt

Testing: hotspot_gc_shenandoah

-Aleksey