[whatwg/streams] ReadableStream: releaseLock cannot be called if read() resolves (#1000)

Or maybe I should say, I cannot find any way cancelling a ReadableStream without running into a synchronization issues.

To cancel a _ReadableStream_ the reader has to release the lock on the stream.
Releasing the [Reader](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/readable-stream.js#L811) requires to call [releaseLock()](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/readable-stream.js#L937).

This call will throw an error of there is an outstanding [read](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/readable-stream.js#L913).



```JavaScript
stream = response.body;  // some ReadableStream;

stream.getReader();
this.pendingRead = this.reader.read();
```
Let's assume the read is still pending here....

At the same time, I would like to get rid of the stream.

Aiming to call [releaseLock()](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/readable-stream.js#L937), when there is no ongoing read I try to used the promise returned by the [read](https://github.com/whatwg/streams/blob/6f94580f6731d1e017c516af097d47c45aad1f56/reference-implementation/lib/readable-stream.js#L913).

```JavaScript
await this.pendingRead; // So let's wait for the read to complete, zzz....

// Should be fine now, right?
this.reader.releaseLock(); // Kaboom: Tried to release a reader lock when that reader has pending read() calls un-settled
```
It looks like, the internal mechanism is resolved **after** the client side of the promise is resolved.

The issue can be demonstrated on Google Chrome, I am not sure that reflects this reference implementation. I use a Readable stream return by [fetch](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API).


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