SoS Kernel init sometimes fails due to race condition

[DavidHuebner]

Describe the bug
When starting the kernel, there is sometimes an error due to a race condition(?) - it happens more frequently in Docker container than on my local machine.

Stacktrace

Exception in callback <TaskWakeupMethWrapper object at 0x7f011f14e7f0>(<Future finis...7263\r\n\r\n'>)
handle: <Handle <TaskWakeupMethWrapper object at 0x7f011f14e7f0>(<Future finis...7263\r\n\r\n'>)>
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
RuntimeError: Cannot enter into task <Task pending name='Task-4' coro=<HTTP1ServerConnection._server_request_loop() running at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/http1connection.py:823> wait_for=<Future finished result=b'GET /kernel...17263\r\n\r\n'> cb=[IOLoop.add_future.<locals>.<lambda>() at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/ioloop.py:688]> while another task <Task pending name='Task-55' coro=<MappingKernelManager.start_kernel() running at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/notebook/services/kernels/kernelmanager.py:176> cb=[IOLoop.add_future.<locals>.<lambda>() at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/ioloop.py:688]> is being executed.
Exception in callback <TaskWakeupMethWrapper object at 0x7f011f9a4bb0>(<Future finis...7263\r\n\r\n'>)
handle: <Handle <TaskWakeupMethWrapper object at 0x7f011f9a4bb0>(<Future finis...7263\r\n\r\n'>)>
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
RuntimeError: Cannot enter into task <Task pending name='Task-5' coro=<HTTP1ServerConnection._server_request_loop() running at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/http1connection.py:823> wait_for=<Future finished result=b'GET /api/co...17263\r\n\r\n'> cb=[IOLoop.add_future.<locals>.<lambda>() at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/ioloop.py:688]> while another task <Task pending name='Task-55' coro=<MappingKernelManager.start_kernel() running at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/notebook/services/kernels/kernelmanager.py:176> cb=[IOLoop.add_future.<locals>.<lambda>() at /home/huebner/src/incubator/IRuta/venv/lib/python3.8/site-packages/tornado/ioloop.py:688]> is being executed.

To Reproduce
Steps to reproduce (does not always fail):

• Copy https://github.com/averbis/IRuta
• Build the docker image: "docker build -t iruta"
• Run the docker image: "docker run -p 8888:8888 iruta"
• Open the notebook "Notebook [IRuta, IPython] - How to combine Ruta and Python"