A crucial factor in the "mismatch" between Shenandoah and GenShen are the concept of Generation and of the "active generation".

The previous task JDK-8325671 introduced into Shenandoah the concept of a ShenandoahGenerationType in support of GenShen.

This one attempts to clean up and smooth out the concept of "active generation" that was introduced in GenShen so that it will fit more cleanly & smoothly into Shenandoah's existing abstractions.

The work will involve some changes to how "active_generation" is used in GenShen, and then will involve a ticket in which that abstraction is then grafted to Shenandoah.