RFR: 8255070: Shenandoah: Use single thread for concurrent CLD liveness test [v3]

[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.

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Replaced naked lock/unlock with MutexLocker

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/777/files
  - new: https://git.openjdk.java.net/jdk/pull/777/files/c46f8d35..f2dded6b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=777&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=777&range=01-02

  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 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