Migrate tokio-core -> tokio 0.2.

[wt]

This should enable a migration to newer version of other deps like reqwest in the future.

[mathias-arm]

Have you looked at #158?

[wt]

I wasn't sure how to fix it. But yet, that is essentially what I want. :)

[wt]

Oh, And I was trying to do it in piecewise moves instead of all at once.

[mathias-arm]

In hindsight, #158 probably could have benefited from smaller steps, but in the end it would be difficult to cherry-pick because the different changes build on each other.

[wt]

I eagerly await the landing of #158 then. Thx. :)

[mbrossard]

158 was merged