BUG Random failures on Appveyor

[tomMoral]

There are several random failures in appveyor.
Let's try to report all of them here:

• timeout in teardown in test_in_callback_submit_with_crash. In this one, it hangs while joining the QueueManagerThread, which is already finished.

• invalid handle in QueueManagerThread for test_worker_timeout. The following traceback can be seen in stderr:

Exception in thread QueueManagerThread:
Traceback (most recent call last):
  File "C:\Python27-x64\Lib\threading.py", line 801, in __bootstrap_inner
    self.run()
  File "C:\Python27-x64\Lib\threading.py", line 754, in run
    self.__target(*self.__args, **self.__kwargs)
  File "C:\projects\loky\loky\process_executor.py", line 681, in _queue_management_worker
    p._worker_exit_lock.release()
WindowsError: [Error 6] The handle is invalid

• timeout in test_worker_timeout. This one might be because the test can be very slow as it need to re-start a lot of worker in the worst case.

[tomMoral]

Another of this kind: here