Use hinshun/cancel for cancellable readers without data being consumed

[hinshun]

Eliminate a lot of code with a proper cancellable reader.

[coryb]

Nice, assuming the cancel readers works reliably this looks like a good improvement.

[coryb]

odd that it says indirect here, it is explicitly used in the proejct...

[hinshun]

The Cancel() method returns bool to indicate whether it was successfully canceled. We could possibly plumb that somehow to the user, but there's no much they can do about it because its platform specific.

[hinshun]

Looks like someone else already hoisted the package, so I'll let them maintain it:
https://github.com/muesli/cancelreader

Need to contrib some PRs there first though.

[hinshun]

My patches were upstreamed to: https://github.com/muesli/cancelreader
And I've updated this PR accordingly.

We should probably test this with downstreams before considering merging this.

[aaronlehmann]

Let's use a timer we can explicitly stop here:

Suggested change

	select {
	case <-readCh:
	case <-time.After(*readTimeout):
	c.cancelReader.Cancel()
	}
	timer := time.NewTimer(*readTimeout)
	select {
	case <-readCh:
	timer.Stop()
	case <-timer.C:
	c.cancelReader.Cancel()
	}

Otherwise, the timers created by time.After will stay active after a read finishes. If reads are happening very fast, this can cause a lot of unnecessary timers to accumulate.

[aaronlehmann]

I think this should be at least a few seconds to avoid flakiness on a heavily loaded system.

[aaronlehmann]

@hinshun: Might be nice to get this one wrapped up. It seemed like a worthwhile improvement.

[aaronlehmann]

LGTM