ENH: Pstream::exchange with Map<Container> data

- dynamic sparse data exchange using Map to hold data and sizes.

  Still uses the personalised exchange paradigm, but with non-blocking
  consensus exchange to obtain the sizes and regular point-to-point
  for the data exchange itself. This avoids an all-to-all but still
  keeps the point-to-point for overlapping communication, data
  chunking etc.