RFR: Concurrent cleanup after partial GC gets accounted in gross pause

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/stats-partial-gross/webrev.01/

Trivial: the scope for ShenandoahGCPhase is off

Testing: eyeballing -Xlog:gc+stats

Thanks,
-Aleksey