Refactor to async / await

[andresilva]

When this crate was initially designed there was no async/await support, as a result the asynchronous logic in this crate is implemented with custom Future impls.

Recently we wanted to make some of the methods on Environment async, e.g. round_data should return a Future as it requires a call to a keystore that may be remote. This small change propagates throughout the crate (e.g. Voter::new needs to become asynchronous) and is not amenable to the current architecture. A redesign to use async/await and not rely on Future implementations would make this change easier to implement.