Update to futures `0.3.0-alpha.18`

[LucioFranco]

Now that rust-lang/futures-rs#1789 has been merged we can update tokio to this new version of futures.

[taiki-e]

Blocked on tokio-rs/loom#35 (I received "failed to select a version" error when trying to do this.)

[carllerche]

We probably want to do #1264 before pushing out another release, otherwise things are going to be pretty painful.

[LucioFranco]

Agreed we should combine