RFR: Refactor worker timings into ShenandoahPhaseTimings

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/stats-worker-times/webrev.01/

Noticed we now have shenandoahPhaseTimings.(hpp|cpp) and shenandoahPhaseTimes.{cpp|hpp}. The latter
does the per-worker accounting, and merges it back to total counters. Moved that into our recently
added file, and did a few renames to capture those bits are indeed about workers.

Does not change the output:
 http://cr.openjdk.java.net/~shade/shenandoah/stats-worker-times/before.txt
 http://cr.openjdk.java.net/~shade/shenandoah/stats-worker-times/after.txt

Testing: hotspot_gc_shenandoah, eyeballing gc+stats

Thanks,
-Aleksey