[15u] RFR (S) 8252648: Shenandoah: name gang tasks consistently

[Aleksey Shipilev]

Original fix:
   https://bugs.openjdk.java.net/browse/JDK-8252648
   https://hg.openjdk.java.net/jdk/jdk/rev/1a839c572dfd

There is a little conflict around ShenandoahConcurrentWeakRootsEvacUpdateTask definition, resolved 
by hand.

15u webrev:
   https://cr.openjdk.java.net/~shade/8252648/webrev.15u.01/

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey