Similar to ZGC, Shenandoah also rendezvous threads to purge stalled states before purge dead class loaders.

Currently, Shenandoah only rendezvous java threads, while ZGC also rendezvous GC threads ( https://github.com/openjdk/jdk/blame/master/src/hotspot/share/gc/z/zGeneration.cpp#L1349-L1350)

I believe Shenandoah should do the same.