constrained topo (remember to rebase all these before merging to master!)
Back to one receive request, prepped for parallel work threads
Tests for the examples in the README
Removed some debug code, made special AnyUniquePtr class for MpiMessage.
unique_ptr instead of shared, no longer relies on first work being in-progress.
Threading model implemented and tested.  A few loose ends to tie up, but it's better than the current version (and more stable)
Merge branch 'worker-threads'