FlatMap overloads with maximum concurrency parameter

[akarnokd]

For issue #2626.

There is no point in adding maxConcurrent to flatMapIterable because each iterable is subscribed to sequentially and run to completion.

[benjchristensen]

👍

[akarnokd]

There is no point in adding maxConcurrent to flatMapIterable because each iterable is subscribed to sequentially and run to completion.

I was wrong with this statement. Depending on the request pattern, reordering may still happen and the downstream may be still slower. Unboundedness of flatMapIterable may still cause excessive source Iterable generation that fills in the tracking arrays inside flatMap.

Having the maxConcurrency parameter is valuable and prevents buffer bloat.