https://github.com/openjdk/jdk/pull/17561 updates the implementation of Shenandoah FreeSet to provide generality required by Generational Shenandoah. After PR17561 is integrated, merge these changes into Generational Shenandoah.

Though the upstream PR17561 was derived from the existing GenShen freeset implementation, the process of reviewing for upstream integration required multiple performance improvements to the implementation, which are now missing from the GenShen implementation.