Because `self.pending` doesn't necessarily get cleaned up in a timely fashion -
rather, only when the user calls `poll_ready()` - it was possible for it to
refer to a stream that has already been closed. This would lead to a panic the
next time that `poll_ready()` was called.
Instead, use an `OpaqueStreamRef`, bumping the refcount.
A change to an existing test is included which demonstrates the issue.
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