Currently, `OFHTTPClient` only supports one in-flight request at a time. This makes it simple to reuse connections with keep-alive and pipeline requests. However, since `OFHTTPClient` is async now, this can be confusing to users. While the intent is to have multiple requests following each other in an async way, the async nature makes users expect they can also perform multiple simultaneous requests.

To make it possible to have multiple in-flight requests at once, state needs to be moved out of `OFHTTPClient` and the logic to reuse connections needs to be reworked entirely.

Changing this should not break the API or ABI, as the way it is being used now with one `OFHTTPClient` per concurrent request will keep working. Hence this change can be implemented post 1.0.