Switch to async/await syntax once stabilized

[gakonst]

https://boats.gitlab.io/blog/post/await-decision-ii/

I’m working actively on drafting a stabilization report to propose stabilizing a minimum viable version of async/await in the 1.37 release of Rust. 1.37 will be released in mid-August, and branches on July 4.

Once that's out, we should get a PR that replaces all the futures handling with async/await calls.

[bstrie]

Blocked on #142. Also note that async/await syntax has since been confirmed for Rust 1.38, releasing on September 26.