implemented Task.runAsyncInterruptibly

I think this resolves #599, in the easiest from, w/o requiring heavy changes to to Future`s signatures. 

I think it covers the use case in streams, and does not broke the clean Future/Task signature. 

@pchiusano, can you take a quick look? 

@larsh Hence this does not change the signature, I believe is binary compatible with 7.0.x series. Do you think I can back-port it to next 7.0.x? Shall I make separate PR to 7.0.x branch? 

Tx 

P.