Begin to use async/await in pants

[stuhood]

See http://areweasyncyet.rs/. Blocked on rust-lang/rust#50547.

I've experimented with this a little bit using the instructions from https://jsdw.me/posts/rust-asyncawait-preview/ ... although I used a slightly older nightly (February 2019), it's clear that there will be a fair number of limitations in place when this stabilizes:

• async/await will not be usable initially in traits (eg impl X for Y), but is usable with inherent methods (eg impl Y)
• async methods may not be recursive (async fn cannot be recursive rust-lang/rust#53690)
• For now, methods with multiple lifetime parameters are not supported: at most one lifetime. (async fn should support multiple lifetimes rust-lang/rust#56238)

I've pushed the branch I was working in over here: master...twitter:stuhood/tokio-async-await ... the first few methods I attempted to adapt ran into the above issues, but I did find a good example to adapt, and the result was easier to read, did fewer allocations, and would require less cloning.

[Eric-Arellano]

Now being used :D