Add async version of jp_server_cleanup fixture

[davidbrochart]

Fixes jupyterlab/jupyter-collaboration#221.

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[davidbrochart]

Actually this is not enough to fix jupyterlab/jupyter-collaboration#221.
Maybe autouse should also be disabled for jp_server_cleanup, and each test should choose the right fixture depending if it's async or not? It seems impossible for the right version to be automatically picked up.