Application.run_async leaking memory

Upon running the example code in examples\prompts\asyncio-prompt.py, I noticed the memory usage climbed rapidly and consistently on my machine -- simply leaving the application sit without doing anything with it would have python.exe using a gigabyte or more of RAM in a few minutes. This was run on a Windows 10 machine running Python 3.8.1.

A bit of investigation later, it appears that the issue may be the underlying app.run_async call is adding thousands of tasks to event loop during its execution. Modifying line 30 of the asyncio-prompt.py example file from its current print statement to print(f'Tasks: {len(asyncio.all_tasks())}') shows that the number of active tasks in the event loop grows very quickly during execution:

This is only an issue with the current master branch version -- it does not occur in prompt-toolkit 3.0.2 from pip, but I figured the issue might still be of use to report.

Start with examples/prompts/asyncio-prompt.py, especially line 30 and the app.run_async call. Reproduce the report with asyncio.all_tasks() while comparing the current master behavior with prompt-toolkit 3.0.2 from pip. Done means the example no longer accumulates tasks or rapidly increases memory while remaining usable.