If you don't need windows support, O'Caml with Jane Street's Core is pretty good. The Async module handles the concurrency bits, and the Pipe module handles the streaming parts.
Note that for now Ocaml is still single threaded so the concurrency is cooperative, similar to how it is in Python or Nodejs. BTW, I'd recommend Lwt instead of Async because its more popular (kind of sad that there are two competing concurrency libs though)