optional no wait on Pstream::exchange

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

src/OpenFOAM/db/IOstreams/Pstreams/OPstream.H

src/OpenFOAM/db/IOstreams/Pstreams/Pstream.H

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

src/OpenFOAM/db/IOstreams/Pstreams/PstreamBuffers.H

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

src/OpenFOAM/db/IOstreams/Pstreams/UIPstream.H

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

src/OpenFOAM/db/IOstreams/Pstreams/UOPstream.H

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

src/Pstream/dummy/UIPread.C

src/Pstream/dummy/UOPwrite.C

src/Pstream/mpi/UIPread.C

src/Pstream/mpi/UOPwrite.C