Use a synchronous ThreadLocalAction for killing other threads

* That way we have a snapshot fo the currently-running fibers and avoid
  e.g. scheduling the action on 2 Fibers of the same thread. See the
  javadoc of Env#submitThreadLocal which say:
  > Already started synchronous actions will block on activation of a new thread.
* Also this means we don't throw the KillException until all
  currently-running fibers have reached the safepoint.
* Note that because of the throw this synchronous action will be exited
  quickly by all threads once all threads have reached the safepoint. So
  there is no concern about a nested safepoint or
  new threads staying blocked a long time.
* This was the only ThreadLocalAction which affects multiple Ruby Threads
  and was asynchronous.

Author: eregon

src/main/java/org/truffleruby/core/thread/ThreadManager.java

@@ -664,7 +664,7 @@ private void doKillOtherThreads() {
 
         context.getSafepointManager().pauseAllThreadsAndExecute(
                 DummyNode.INSTANCE,
-                new SafepointAction("kill other threads for shutdown", predicate, true, false) {
+                new SafepointAction("kill other threads for shutdown", predicate, true, true) {
                     @Override
                     public void run(RubyThread rubyThread, Node currentNode) {
                         rubyThread.status = ThreadStatus.ABORTING;