RFR: 8305670: Performance regression in LockSupport.unpark with lots of idle threads

[Daniel D. Daugherty]

Address the performance regression in `LockSupport.unpark()` with the following changes:
- Add a `quick_mode` parameter to `ThreadsListHandle::cv_internal_thread_to_JavaThread()` that tells the conversion function to skip searching the ThreadsList for the target `JavaThread*`.
- Update `Unsafe_Unpark()` to verify that `java_lang_Thread::thread(thread_oop) != nullptr` both before and after the creation of the ThreadsListHandle; this verification that a `JavaThread*` is saved in the `java.lang.Thread` object allows us to know that the target `JavaThread*` is protected by the ThreadsListHandle without searching it.
- Move the storing of the `JavaThread*` in the `java.lang.Thread` object until after the `JavaThread*` is added to the system ThreadsList.
- This change in the order permits the check of `java_lang_Thread::thread(thread_oop) != nullptr` before the creation of the ThreadsListHandle to have the meaning that we need for a `JavaThread*` that is in the early stages of running.
- The check `java_lang_Thread::thread(thread_oop) != nullptr` that happens after the creation of the ThreadsListHandle permits the meaning that we need for a `JavaThread*` that is in the end stages of running.
- So the combination of the two `java_lang_Thread::thread(thread_oop) != nullptr` checks allows us to safely skip the ThreadsList search in `ThreadsListHandle::cv_internal_thread_to_JavaThread()`.

Additional changes:
- Remove the `EnableThreadSMRExtraValidityChecks` option since that option implies that the ThreadsList search is optional in all situations which is not the case. Also remove comments that mention the option.
- Update the comments in `ThreadsListHandle::cv_internal_thread_to_JavaThread()` to clarify what `java_lang_Thread::thread(thread_oop) != nullptr` means in the JavaThread's lifecycle and that function's attempts to convert a `jthread` into a `JavaThread*` that's known to be protected by the ThreadsListHandle.

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Commit messages:
 - 8305670.with-rehn-quick-mode.cr0.patch

Changes: https://git.openjdk.org/jdk/pull/13519/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13519&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305670
  Stats: 73 lines in 6 files changed: 40 ins; 12 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/13519.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13519/head:pull/13519

PR: https://git.openjdk.org/jdk/pull/13519