Fix issue with HttpAsyncClient blocking after the first post / after opening two get streams

[marvk]

HttpAsyncClient seems to be never sending requests after some requests have been send. This is possibly due to a threading issue.

For example, after opening the event stream, then accepting a game via POST and opening the game stream, POSTed moves will not be send. As soon as the game stream is closed, e.g. via forfeiting the game on lichess, the request comes through (with a 400 status code).