RFR (S) 8252648: Shenandoah: name gang tasks consistently

[Aleksey Shipilev]

RFE:
   https://bugs.openjdk.java.net/browse/JDK-8252648

Current uses of AbstractGangTask("...") are all over the place, sometimes they are misleading. Need 
to clean this up.

Webrev:
   https://cr.openjdk.java.net/~shade/8252648/webrev.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey