Make I/O threadsafe

[brson]

uv-based I/O is bound to a specific scheduler but tasks are not. This means that when a task goes to perform I/O it needs to check whether it is on the correct scheduler and if not, send itself as a message to the correct scheduler. Types that implement rtio traits will need to contain a SchedHandle to their home scheduler.

[huonw]

Triage 2013-08-05: grep seems to suggest that this (rtio-trait-implementors having a SchedHandle) isn't the case yet. (Although there has been a lot of work in the last 2 months, so the design plan may've changed.)

Is this still an issue, @brson or @bblum?

[anasazi]

I believe this has been resolved by #8631. Can we close @brson?

[alexcrichton]

We now having the HomingIO trait, closing this.