Flush is broken when ordering enabled and multiple batches present

[kimjoar]

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Fixes #<issue_number_goes_here> 🦕

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flush is currently broken when you have ordering + multiple batches when the flush occurs, as publishDrain gets called with a callback that's dropped in the publish implementation of OrderedQueue. The callback is not passed on to beginNextPublish, so the flush will never complete as it's waiting for the callback to be called. Looks like this came in with #1691.

Haven't looked into tests etc. Let me know what you think makes sense, if this is interesting to merge.

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[kimjoar]

I think I've realized that flush shouldn't be used at all, and breaks ordering, if I'm not mistaken. Not clear from the docs.