Support maintaining order of chunks

[almost]

As an option, not appropriate for a lot of use cases.

It would need to have a limit on the output buffer (the list of chunks that have arrived out of order and are waiting for the the one in front of them to be ready) otherwise it would be easy for it to use up all available memory if a particular chunk took a long time to process. This probably wouldn't be the same as the maxConcurrency option, this should continue to mean how many things can be being processed at a given time.