Round timer durations up to 0

[djspiewak]

IO.sleep(-1.millis) should equal IO.sleep(0.millis), which in turn should equal IO.cede. It's probably easiest to play this trick in IOFiber rather than tracking down all the various places we call sleep.

[manuelcueto]

I can take this one 👍

[djspiewak]

@manuelcueto Any updates on this? I think this is the last issue remaining before 3.5.0.