ENH: support independent handling of MPI requests (#2674)

- UPstream::Request wrapping class provides an opaque wrapper for
  vendor MPI_Request values, independent of global lists.

ENH: support for MPI barrier (blocking or non-blocking)
ENH: support for MPI sync-send variants

STYLE: deprecate waitRequests() without a position parameter

- in many cases this can indicate a problem in the program logic since
  normally the startOfRequests should be tracked locally.

src/OpenFOAM/db/IOstreams/Pstreams/UIPstreamBase.C

@@ -6,7 +6,7 @@
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
     Copyright (C) 2011-2015 OpenFOAM Foundation
-    Copyright (C) 2017-2022 OpenCFD Ltd.
+    Copyright (C) 2017-2023 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -146,7 +146,7 @@ inline Foam::Istream& Foam::UIPstreamBase::readString(std::string& str)
 
 Foam::UIPstreamBase::UIPstreamBase
 (
-    const commsTypes commsType,
+    const UPstream::commsTypes commsType,
     const int fromProcNo,
     DynamicList<char>& receiveBuf,
     label& receiveBufPosition,