Deregister Eventeds before dropping

[cramertj]

If tokio-rs/mio#753 is implemented, then tokio-core will have to deregister Eventeds rather than expecting them to be deregistered on Drop. However, I believe we can make this change now (backwards-compatibly).

[alexcrichton]

I sort of disagree, but we can discuss on tokio-rs/mio#753