mpi: Squeeze back down from 28 to 23 tag bits to satisfy Cray MPI

1. Add 8 bytes to data to each request message including the dimensions
   (parent to child symmetry and child dimension) and a unique response tag.
2. Respond using the response tag.
3. Remove the symmetry from all tags, saving 3 bits.
4. Note that there are fewer lines than blocks, saving at least 2 more.