BUG: incorrect chunk handling in Pstream::exchange (fixes #2375)

- used Pstream::maxCommsSize (bytes) for the lower limit when sending.
  This would have send more data on each iteration than expected based
  on maxCommsSize and finish with a number of useless iterations.

  Was generally not a serious bug since maxCommsSize (if used) was
  likely still far away from the MPI limits and exchange() is primarily
  harnessed by PstreamBuffers, which is sending character data
  (ie, number of elements and number of bytes is identical).

applications/test/parallel-chunks/Make/files

@@ -0,0 +1,3 @@
+Test-parallel-chunks.C
+
+EXE = $(FOAM_USER_APPBIN)/Test-parallel-chunks

applications/test/parallel-chunks/Make/options

@@ -0,0 +1,2 @@
+/* EXE_INC = */
+/* EXE_LIBS = */