Migrate to async/await

[thomaseizinger]

async/await landed on stable Rust in version 1.39 on 07.11.2019!

This epic tracks all issues that are related with moving our codebase to async/await.

Current setup

• You can use async/await within the cnd and btsieve crates (those have the necessary compatibility layers setup that are required to have new and old Futures work together)
• Some code parts are already making heavy use of async/await (f.e. btsieve internals)

What do we need to migrate everything (without a compatibility layer)?

• libp2p needs to update to std-futures (Update libp2p to version 0.16 #2072)
• warp needs to update to std-futures (see update to std::future::Future seanmonstar/warp#265 and Update to hyper 0.13/tokio 0.2 seanmonstar/warp#336)
• tokio needs to update to std-futures (alternatively we could switch to async-std)

Benefits

Converting the codebase to async/await is expected to bring the following benefits:

Less code

Writing asynchronous code with async/await instead of using combinators on Future needs significantly less LoC to express the same functionality.

Better ergonomics

Usage of combinators often requires cloning of references. This boilerplate is going away with async/await.

Better organization of code

With async/await, code reuse goes back to declaring a function (an async one). On the other hand, code reuse through manual creation of Futures requires the definition of structs.

More concretely, moving to async/await should allow for an easier way of reusing code because it is just functions (again).