job:  #589  Adding test model that exercises returning strings in the face of multi-threading.  There are 3 or 4 independent groups of classes.  upper and lower are a pair; ping and pong are a pair, and spin and spun can be paired or run in their own threads.
Merge pull request #1 from cortlandstarrett/589_stringtest

job:  #589  Adding test model that exercises returning strings in the fa...