STYLE: communication name "buffered" instead of "blocking"

- "buffered" corresponds to MPI_Bsend (buffered send),
  whereas the old name "blocking" is misleading since the
  regular MPI_Send also blocks until completion
  (ie, buffer can be reused).

ENH: IPstream::read() returns std::streamsize instead of label (#3152)

- previously returned a 'label' but std::streamsize is consistent with
  the input parameter and will help with later adjustments.

- use <label> instead of <int> for internal accounting of the message
  size, for consistency with the underyling List<char> buffers used.

- improve handling for corner case of IPstream receive with
  non-blocking, although this combination is not used anywhere