[jdk11u-dev] RFR: 8252648: Shenandoah: name gang tasks consistently

[Aleksey Shipilev]

Unclean backport, because Shenandoah 11u does not have concurrent weak reference processing and concurrent class unloading.

Additional testing:
 - [x] Linux x86_64 fastdebug `hotspot_gc_shenandoah`
 - [x] Inspecting `AbstractGangTask` uses in Shenandoah code

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Commit messages:
 - Backport a1176dc8c9e53e68e1af70b0c737c158156ba8af

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/104/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=104&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252648
  Stats: 20 lines in 6 files changed: 0 ins; 0 del; 20 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/104.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/104/head:pull/104

PR: https://git.openjdk.java.net/jdk11u-dev/pull/104