This removes all the non-async code from the aiohttp generator:

• all the methods that should be asynchronous are marked as async
• the async_req parameter is gone; calls are directly awaitables now
• the async calls into a thread pool are gone and the thread pool is gone too (now useless)

Closes: #15824
Closes: #5539
Related: #763
Related: #3696

The code is extracted from #15878, readjusted to remove the typing changes and fixed for the aiohttp client tests to pass.

@krjakbrjak @wing328

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