Concurrent class unloading and heap iteration can run concurrently, they both can iterate nmethods.

Current implementation only allow one iterator, but there is no reason for not allowing multiple iterators.

I am surprised that we have yet seen test failures. But after I twisted concurrent weak root processing a bit, I saw the failure.

# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/zgu/ws/jdk/src/hotspot/share/gc/shenandoah/shenandoahNMethod.cpp:471), pid=1970, tid=1977
# assert(!iteration_in_progress()) failed: Already in progress
#
# JRE version: OpenJDK Runtime Environment (15.0) (fastdebug build 15-internal+0-adhoc.zgu.jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 15-internal+0-adhoc.zgu.jdk, mixed mode, tiered, shenandoah gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x1642304] ShenandoahNMethodTable::snapshot_for_iteration()+0xb4
#