Cannot destroy threadpool worker threads after fork

[jamadden]

• gevent version: 20.5.1

If the threadpool worker ran a task that initialized a hub, because after the fork the thread is dead and we try to destroy from a different thread. This is a bit of a regression introduced by #1601

Traceback (most recent call last):
 File "//gunicorn-19.9.0-py2.7.egg/gunicorn/arbiter.py", line 203, in run
   self.manage_workers()
 File "//gunicorn-19.9.0-py2.7.egg/gunicorn/arbiter.py", line 545, in manage_workers
   self.spawn_workers()
 File "//gunicorn-19.9.0-py2.7.egg/gunicorn/arbiter.py", line 616, in spawn_workers
   self.spawn_worker()
 File "//gunicorn-19.9.0-py2.7.egg/gunicorn/arbiter.py", line 567, in spawn_worker
   pid = os.fork()
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/os.py", line 476, in fork
   return fork_and_watch(*args, **kwargs)
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/os.py", line 431, in fork_and_watch
   pid = fork()
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/os.py", line 198, in fork_gevent
   reinit()
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/hub.py", line 357, in reinit
   getattr(obj, '_on_fork', lambda: None)()
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/threadpool.py", line 384, in _on_fork
   worker.cleanup(worker._hub_of_worker)
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/threadpool.py", line 194, in cleanup
   hub_of_worker.destroy(True)
 File "//gevent-20.5.1-py2.7-linux-x86_64.egg/gevent/hub.py", line 787, in destroy
   self.throw(GreenletExit)
error: cannot switch to a different thread