Update to tokio 0.2

[seanmonstar]

• Blocked on h2: https://github.com/hyperium/h2/milestone/2

[liufuyang]

looks like the blocker is gone now? :)

[bikeshedder]

The h2 change was just commited to the code repository and no new release uploaded to crates.io. 😞

[alex]

A follow up change is needed to h2 to restore all behavior: hyperium/h2#429

Please keep in mind that in the US it's Thanksgiving weekend, so many folks will not be looking at github, instead spending time with friends and family and over eating.

[bstrie]

hyperium/h2#429 has merged, though for the remaining blocker of publishing a new h2 release let's utilize threads in the relevant repo: hyperium/h2#430

[seanmonstar]

Yep, h2 updated its tokio dependency, there's one last remaing thing there: we need a new http release since tokio uses a newer bytes.

This is my highest priority this week!