- the maxCommsSize variable is used to 'chunk' large data transfers
  (eg, with PstreamBuffers) into a multi-pass send/recv sequence.

  The send/recv windows for chunk-wise transfers:

      iter    data window
      ----    -----------
      0       [0, 1*chunk]
      1       [1*chunk, 2*chunk]
      2       [2*chunk, 3*chunk]
      ...

  Since we mostly send/recv in bytes, the current internal limit
  for MPI counts (INT_MAX) can be hit rather quickly.

  The chunking limit should thus also be INT_MAX, but since it is
  rather tedious to specify such large numbers, can instead use

      maxCommsSize = -1

  to specify (INT_MAX-1) as the limit.
  The default value of maxCommsSize = 0 (ie, no chunking).

Note
~~~~
  In previous versions, the number of chunks was determined by the
  sender sizes. This required an additional MPI_Allreduce to establish
  an overall consistent number of chunks to walk. This additional
  overhead each time meant that maxCommsSize was rarely actually
  enabled.

  We can, however, instead rely on the send/recv buffers having been
  consistently sized and simply walk through the local send/recvs until
  no further chunks need to be exchanged. As an additional enhancement,
  the message tags are connected to chunking iteration, which allows
  the setup of all send/recvs without an intermediate Allwait.

ENH: extend UPstream::probeMessage to use int64 instead of int for sizes