Shenandoah: Improve handshake closure labels

Currently, Shenandoah has a few handshakes that have not very clear names, "ShenandoahRendezvous". Would be good to make them more explicit.

```
Event: 2.593 Executing VM operation: Shenandoah Init Marking
Event: 2.594 Executing VM operation: Shenandoah Init Marking done
Event: 2.599 Executing VM operation: HandshakeAllThreads (Shenandoah Flush SATB Handshake)
Event: 2.599 Executing VM operation: HandshakeAllThreads (Shenandoah Flush SATB Handshake) done
Event: 2.599 Executing VM operation: HandshakeAllThreads (Shenandoah Flush SATB Handshake)
Event: 2.599 Executing VM operation: HandshakeAllThreads (Shenandoah Flush SATB Handshake) done
Event: 2.599 Executing VM operation: Shenandoah Final Mark and Start Evacuation
Event: 2.600 Executing VM operation: Shenandoah Final Mark and Start Evacuation done
Event: 2.600 Executing VM operation: HandshakeAllThreads (ShenandoahRendezvous)
Event: 2.600 Executing VM operation: HandshakeAllThreads (ShenandoahRendezvous) done
Event: 2.604 Executing VM operation: HandshakeAllThreads (ShenandoahRendezvous)
Event: 2.604 Executing VM operation: HandshakeAllThreads (ShenandoahRendezvous) done
Event: 2.605 Executing VM operation: CleanClassLoaderDataMetaspaces
Event: 2.606 Executing VM operation: CleanClassLoaderDataMetaspaces done
Event: 2.606 Executing VM operation: Shenandoah Init Update References
Event: 2.606 Executing VM operation: Shenandoah Init Update References done
Event: 2.611 Executing VM operation: HandshakeAllThreads (Shenandoah Update Thread Roots)
Event: 2.611 Executing VM operation: HandshakeAllThreads (Shenandoah Update Thread Roots) done
Event: 2.611 Executing VM operation: Shenandoah Final Update References
Event: 2.611 Executing VM operation: Shenandoah Final Update References done
```