Remove need for ack messages in shuffler implementation

[wence-]

In the allgather implementation, we decided that it was OK to commit to location of data buffers as soon as they were ready, rather than doing a "I have some data/OK I am ready for it" syn-ack round trip.

We should do the same in the shuffler: this would simplify the logic in the event loop and remove one round trip of messages.

[pentschev]

Just FYI, I'm still planning to do this as part of #437, just didn't have much time lately to push that forward. I'll re-assign it to myself then, but if you think it needs to be done earlier feel free to do it on a PR of its own instead.

[pentschev]

This is being addressed in #535, specifically ac830c0 . I decided to make this change in a follow-up PR to #437 because any behavior change like performance will be more easily trackable this way.