io: support zero size messages, consistently.

- for single communication calls (write, read, write_notify,
  read_notify) ie. does not include lists or passive communication
  yet.

- minimum communication size was 1 byte but there are cases where an
  application considers the size zero, mostly for convenience.

- if this was not verified (with debug lib) the behavior on IB and TCP
  was different, allowed and error, respectively. Also zero-size read
  operation would break a queue on IB.

- add tests for each communication call.