A future-based drain for the asynchronous future

[nagisa]

With futures and async coming into the rust standard library, proper, perhaps slog-async should grow some support for a Future to act as the other end of a Drain? This would allow one to spawn all of the logging on an executor which will inevitably exist in any Rust project of 2020.

This is what code would end up looking like:

let (drain, future) = slog_async::Something::new();
runtime.spawn(future); // don’t have to think about this again, and runs on the same runtime as everything else.
let logger = slog::Logger::root(drain, ...);

Implementation wise Drain will likely become the sender end of some async-friendly implementation of channel, whereas the future would be implemented in a way resembling this:

async {
    loop { 
        let message = async_channel.pop().await;
        some_fd.poll_write(..., message).await;
    }
}

Sadly, this likely cannot be done in a composable manner with other already existing drains, as those are synchronous.