When Shenandoah finds enough garbage in regions with _no live objects_, it can skip the evacuation and update-references phases. This is called an abbreviated cycle. Currently, such cycles take three safepoints. The last of these is known as 'Final Roots'. This change replaces the safepoint, with a handshake operation. For all modes, this phase turns off the WEAK_ROOTS flag (this is similar to the already removed Init Update Refs safepoint). For the generational mode, there is additional work, but this can be done on the same handshake.