RFR (S) 8243573: Shenandoah: rename GCParPhases and related code

[Roman Kennke]

Looks good.

Thanks,
Roman

> RFE:
>   https://bugs.openjdk.java.net/browse/JDK-8243573
> 
> ShenandoahPhaseTimings::GCParPhases is awkward: it is plural and it carries unnecessary "GC" prefix.
> Should be just ShenandoahPhaseTimings::ParPhase. This also matches the local variables "par_phase"
> to be used to store it, in contrast with just "phase" for ShenandoahPhaseTimings::Phase.
> 
> Webrev:
>   https://cr.openjdk.java.net/~shade/8243573/webrev.01/
> 
> Testing: hotspot_gc_shenandoah
>