RFR: 8259962: Shenandoah: task queue statistics is inconsistent after JDK-8255019

Tue Jan 19 10:36:03 UTC 2021

I believe `ShenandoahSTWMark` misses the TQ stats reset after the takeover from concurrent cycle. See the stack trace and events info in the bug.

Additional testing:
 - [x] Linux x86_64, failing tests now pass
 - [x] Linux x86_64 `hotspot_gc_shenandoah`
 - [ ]  Linux x86_64 `tier1` with Shenandoah

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Commit messages:
 - 8259962: Shenandoah: task queue statistics is inconsistent after JDK-8255019

Changes: https://git.openjdk.java.net/jdk/pull/2141/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2141&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259962
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2141.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2141/head:pull/2141

PR: https://git.openjdk.java.net/jdk/pull/2141