Shenandoah: Handshake threads between weak-roots and reset phases

Preliminary summary, please adjust as appropriate.

#
# A fatal error has been detected by the Java Runtime Environment:
#
# Internal Error (/home/jenkins/workspace/nightly/jdk-jdk/src/hotspot/share/runtime/jniHandles.inline.hpp:63), pid=47747, tid=47947
# assert(external_guard || result != __null) failed: Invalid JNI handle
#
# JRE version: OpenJDK Runtime Environment (16.0) (fastdebug build 16-internal+0-adhoc.jenkins.jdk-jdk)
# Java VM: OpenJDK 64-Bit Server VM (fastdebug 16-internal+0-adhoc.jenkins.jdk-jdk, mixed mode, sharing, tiered, compressed oops, shenandoah gc, linux-amd64)
# Problematic frame:
# V [libjvm.so+0x8700b4] JNIHandles::resolve_non_null(_jobject*)+0x104

Stack: [0x00007f52c38f1000,0x00007f52c39f2000], sp=0x00007f52c39f07a0, free space=1021k
Native frames: (J=compiled Java code, A=aot compiled Java code, j=interpreted, Vv=VM code, C=native code)
V [libjvm.so+0x8700b4] JNIHandles::resolve_non_null(_jobject*)+0x104
V [libjvm.so+0xe1df5f] JVM_IHashCode+0xbf
J 38 java.lang.Object.hashCode()I java.base@16-internal (0 bytes) @ 0x00007f53145292f1 [0x00007f53145291e0+0x0000000000000111]
J 295 c2 gc.stress.gclocker.Filler.<init>()V (87 bytes) @ 0x00007f531454a3c0 [0x00007f5314549e40+0x0000000000000580]
J 322 c2 gc.stress.gclocker.MemoryUser.load()V (63 bytes) @ 0x00007f5314557614 [0x00007f5314557300+0x0000000000000314]
j gc.stress.gclocker.MemoryUser.run()V+27
j java.lang.Thread.run()V+11 java.base@16-internal
v ~StubRoutines::call_stub
V [libjvm.so+0xca1059] JavaCalls::call_helper(JavaValue*, methodHandle const&, JavaCallArguments*, Thread*)+0x719
V [libjvm.so+0xca6771] JavaCalls::call_virtual(JavaValue*, Handle, Klass*, Symbol*, Symbol*, Thread*)+0x581
V [libjvm.so+0xe1aa19] thread_entry(JavaThread*, Thread*)+0x89
V [libjvm.so+0x16d922f] JavaThread::thread_main_inner()+0x21f
V [libjvm.so+0x16ddb56] Thread::call_run()+0xf6
V [libjvm.so+0x1304136] thread_native_entry(Thread*)+0x116

Heap:
Shenandoah Heap
 1500M total, 1500M committed, 765M used
 3000 x 512K regions
Status: not cancelled
Reserved region:
 - [0x00000000a2400000, 0x0000000100000000)
Collection set:
 - map (vanilla): 0x0000000000011448
 - map (biased): 0x0000000000010000