S2C_CHALLENGE sends wrong bytes with Async

[Glitchvid]

When using an Async A2S client and having multiple futures in-flight (using buffered_unordered), any S2C_CHALLENGE responses will just use the most recent challenge bytes received from any server, sending the wrong response.

Client -> b'T' -> Server α
Server α -> b'A' [bytes α] -> Client
Client -> b'T' [bytes α] -> Server β
Server β -> b'A' [bytes β] -> Client
Client -> b'T' [bytes β] -> Server α
etc...

Currently I'm getting around this by having a client pool and using each client exclusively for each buffered_unordered batch; which isn't ideal since the API design allows and encourages using a shared client to make concurrent A2S requests.