The affinity router prematurely closes southbound streams before sendMsg is complete

The affinity router occasionally closes the backend streams before both sendMsg calls have completed. When this happens one of the 2 redundant backends would never get the RPC. A mechnism is required to guarantee that the grpc.sendMsg() has completed before the stream is closed. Waiting for the grpc.sendMsg() call to return is not in itself sufficient to guarantee the message has been sent.