Re: [whatwg/streams] Close should be idempotent (#1136)

> sometimes a stream would be closed by the underlying runtime due to client disconnect, and sometimes it wouldn't

If you're the one implementing the source of a `ReadableStream`, you should have exclusive ownership over the `ReadableStreamDefaultController`, so only your code should be able to call `controller.close()`. The only way a consumer of your stream can cause it to become closed, is when the consumer cancels the stream. You should handle this case by implementing a `cancel()` method on the source.

On the other hand, if the underlying runtime is actually calling `controller.close()` without your knowledge... then that sounds like a bad runtime behavior. It should probably cancel the stream instead to clean it up.

> The same logic applies to `.cancel()` and other forms of deinit/abort/cleanup, but I haven't checked if those have the same issues.

Calling `cancel()` a second time does nothing (see step 2 of [ReadableStreamCancel](https://streams.spec.whatwg.org/#readable-stream-cancel)).

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