RFR: 8255070: Shenandoah: Use single thread for concurrent CLD liveness test [v2]

[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:

  Reduce duplicated code

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/777/files
  - new: https://git.openjdk.java.net/jdk/pull/777/files/5b69d7bb..c46f8d35

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=777&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=777&range=00-01

  Stats: 50 lines in 2 files changed: 14 ins; 25 del; 11 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