Use an async kubernetes client

[sbidoul]

This is a long term improvement.

We currently use the official python kubernetes client for which we have simple async wrappers.

Using an asyncio compatible client would have some benefits such as avoiding the stop/restart difficulties we have due to the use of asyncio.run_in_executor.

An interesting project I'm following is lightkube, but it is not ready yet.

In any case, we need to progress on tests (#3 and #4) before touching this one.