Very slow kernel (~30s) restart with asyncio and ProcessPoolExecutor

Thanks to the great work by @minrk in https://github.com/ipython/ipykernel/pull/290 restarting a kernel cleans everything up correctly.

However, when the `ProcessPoolExecutor` is still running the kernel restart is very slow (~30 seconds vs 2 seconds normally).

```python
import concurrent.futures
import asyncio

executor = concurrent.futures.ProcessPoolExecutor()
ioloop = asyncio.get_event_loop()

def simple_function(x):
import time
time.sleep(10000)
return x

tsk = ioloop.run_in_executor(executor, simple_function, 0)
```