flatMapIterable with maxConcurrent

[benjamindonze]

As far as I saw flatMapIterable doesn't have maxConcurrent parameter and thus doesn't offer a way to apply backpressure. In order to align it with other flatMap would be good to add an overload with maxConcurrent.

What do you think?

[akarnokd]

PR is welcome.

[benjamindonze]

Although you stated here that this was useless #2627 . Is it really the case?
I believe with current implementation this would be usefull to control backpressure.

[akarnokd]

I was wrong back then.

[akarnokd]

Closing via #3722