RFR: 8335356: Shenandoah: Improve concurrent cleanup locking

[Y. Srinivas Ramakrishna]

On Thu, 26 Sep 2024 18:56:20 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Not clean, but conflicts were easy to resolve.

This looks good. Would be good to backport this into JDK 21u-dev directly too.
(cc @pengxiaolong)

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

Marked as reviewed by ysr (Committer).

PR Review: https://git.openjdk.org/shenandoah-jdk21u/pull/106#pullrequestreview-2332206957