function concatMap (f, stream) {
  return mergeMapConcurrently(f, 1, stream)
}
Map each value in stream to a new stream, and concatenate them all
stream:              -a---b---cX
f(a):                 1-1-1-1X
f(b):                        -2-2-2-2X
f(c):                                -3-3-3-3X
stream.concatMap(f): -1-1-1-1-2-2-2-2-3-3-3-3X
@param {function(x:*):Stream} f function to map each value to a stream
@param {Stream} stream
@returns {Stream} new stream containing all events from each stream returned by f