RFR: 8264123: add ThreadsList.is_valid() support

[Daniel D.Daugherty]

ThreadsLists need an is_valid() function and checks in various
places to help catch bugs where a ThreadsList is dangling.

Other minor changes:
- change raw `_threads_hazard_ptr` access to used `get_threads_hazard_ptr()`.
- `get_threads_hazard_ptr()` should be `const`.
- fix a couple of old typos.

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Commit messages:
 - 8264123.00.patch

Changes: https://git.openjdk.java.net/jdk/pull/3255/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3255&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264123
  Stats: 60 lines in 4 files changed: 48 ins; 0 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3255.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3255/head:pull/3255

PR: https://git.openjdk.java.net/jdk/pull/3255