Support for non-preserving mapping

Thanks for the awesome utility function. I was wondering if it would be possible to provide a similar `concurrentMapM_` **without** enforcing ordering, that is, yield all elements as soon as possible without care for their order.

I can do this in [streamly](https://hackage.haskell.org/package/streamly) using `parMapM` (see https://hackage.haskell.org/package/streamly-0.10.1/docs/Streamly-Internal-Data-Stream-Prelude.html#v:parMapM) but I enjoy working with Conduit and I'd rather keep using it than migrating to a different ecosystem altogether.