RFR: Renaming ShenandoahParallelEvacuationTask to ShenandoahConcurrentEvacuationTask

[Zhengyu Gu]

Please review this trivial renaming.

ShenandoahParallelEvacuationTask is used during concurrent evacuation 
phase, will look awkward once plugin jfr events, as it should generate 
concurrent event.

Separating this change to individual patch to easy backport.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/rename_evac_task/webrev.00/


Test:

   tier2_gc_shenandoah

Thanks,

-Zhengyu