new method concatMapIterable

[benjamindonze]

Hello,

I think it would be nice to have an concatMapIterable method beeing the counterpart of flatMapIterable.
Indeed concatMapIterable would seems to me the best and safest way to flatten a Observable<List> -> Observable

Problem with flatMapIterable is that as flatMap use by default an unbounded buffer and unlimited source subscription. I recently had the problem having a producer producing faster than a consumer.
I naively used flatMap(t->Observable.from(t)) or flatMapIterable to flatten the produced list wich always lead to OutOfMemory.
concatMap on the other way doesn't have this issue and thus would be safer to use to flatten Observable for which you don't have implementation detail.

I'm quite new with RxJava so please tell me if I totaly missed the point.

[akarnokd]

PRs are welcome, but note that some are concerned with the API surface area and may not want a convenience overload such as this.

[akarnokd]

Closing via #3718