Fix stream open futures sometimes waiting on the wrong notification

[Ralith]

After a spurious wakeup, Connection::open_uni and open_bi would switch to waiting for notifications about incoming streams, rather than the intended notification about new stream ID flow control budget. This was probably a copy-paste error from poll_accept.

Let's get a patch release out as soon as this is merged.

[Ralith]

(linter complaints addressed in #1439)

[djc]

How hard would it be to add a test for this?

[Ralith]

Science says: not hard.