Shenandoah phase timing tracking and JFR event supporting are inconsistent, as JFR event supporting was added gradually.

This cleanup intends to better define the tracking behaviors by different classes, also added missing JFR worker events.

ShenandoahGCPhaseTiming: only tracks Shenandoah specific timing, but does not emit JFR event.

ShenandoahPausePhase, ShenandoahConcurrentPhase and ShenandoahGCPhase: track Shenandoah specific timings and also emit JFR events.