Investigate async traits

[greg-szabo]

Not very important right now, but just pointing out that async methods are somewhat supported on stable Rust via the async-trait crate. This crate sometimes yield weird compile-time errors and incurs a boxing of the returned Future but has otherwise worked pretty well for us in the past.

Originally posted by @romac in #174 (comment)

[romac]

We already use the async-trait crate in many places in tendermint-rs and in ibc-rs. I think we can close this as it's worked pretty well for us so far.