(Related to #2351/r18400) resolve potential deadlocking issue in state variable interpolation (caused by processes doing Send--then--Recv when messages are bigger than local buffers).

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@18409 b5f69653-2bf5-0310-8201-b0dafb8e1405