RFR: 8255070: Shenandoah: use single thread for concurrent CLD liveness test

[Zhengyu Gu]

Since the concurrent liveness test does not touch any oops, there is no point to use multi-thread for the task, they
just duplicate the work.

Also, multi-thread version requires to acquire ClassLoaderGraph_lock early, which is deadlock prone.

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Commit messages:
 - Fix include
 - 8255070: Shenandoah: use single thread for concurrent CLD liveness test

Changes: https://git.openjdk.java.net/jdk/pull/777/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=777&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255070
  Stats: 31 lines in 3 files changed: 21 ins; 1 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/777.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/777/head:pull/777

PR: https://git.openjdk.java.net/jdk/pull/777