RFR: 8261921: ClassListParser::current should be used only by main thread

[Ioi Lam]

During -Xshare:dump, ClassListParser::current() should be used only by the main thread, which has created the only ClassListParser instance. Accessing it from other threads could cause intermittent failures. We observed this only on certain hosts with -Xcomp.

The fix is to check for `ClassListParser::is_parsing_thread()` before calling `ClassListParser::current()`. After the fix, I can no longer reproduce the crash.

I also did some renaming and comment cleaning.

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Commit messages:
 - 8261921: ClassListParser::current should be used only by main thread

Changes: https://git.openjdk.java.net/jdk/pull/2619/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2619&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261921
  Stats: 43 lines in 5 files changed: 26 ins; 7 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2619.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2619/head:pull/2619

PR: https://git.openjdk.java.net/jdk/pull/2619