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

applications/test/parallel-comm1/Test-parallel-comm1.C

@@ -62,7 +62,7 @@ scalar sumReduce
                 scalar procValue;
                 UIPstream::read
                 (
-                    UPstream::commsTypes::blocking,
+                    UPstream::commsTypes::buffered,
                     proci,
                     reinterpret_cast<char*>(&procValue),
                     sizeof(scalar),
@@ -79,7 +79,7 @@ scalar sumReduce
             {
                 UOPstream::write
                 (
-                    UPstream::commsTypes::blocking,
+                    UPstream::commsTypes::buffered,
                     proci,
                     reinterpret_cast<const char*>(&sum),
                     sizeof(scalar),
@@ -93,7 +93,7 @@ scalar sumReduce
             {
                 UOPstream::write
                 (
-                    UPstream::commsTypes::blocking,
+                    UPstream::commsTypes::buffered,
                     UPstream::masterNo(),
                     reinterpret_cast<const char*>(&localValue),
                     sizeof(scalar),
@@ -105,7 +105,7 @@ scalar sumReduce
             {
                 UIPstream::read
                 (
-                    UPstream::commsTypes::blocking,
+                    UPstream::commsTypes::buffered,
                     UPstream::masterNo(),
                     reinterpret_cast<char*>(&sum),
                     sizeof(scalar),