+str #15105 Add Flow tee

NOT READY. Before completing (with more tests) I would like to verify that this is the right signature and semantics. 

@sirthias, please take a look. Note that it only waits for request from the `other` consumer. The reason why I did not let it wait for request from downstream consumer is that it can be multiple downstream consumers and then it would have to wait for at least one. I can change that, but this might be enough for your coordination of the 2 streams. Not sure what is most intuitive.