[streams] Define valid queuing strategy (#427)

As an author- and other-spec-facing requirement, we should define valid queuing strategies. I think the requirements are:

- have size() methods and highWaterMark properties.
- size() is pure (gives the same result for the same input every time)
- size() never returns NaN, +Infinity, or negative
- highWaterMark is not NaN and not < 0
- (maybe) highWaterMark does not change over time?
- (maybe) size never throws an exception?

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/streams/issues/427