Define a Drain::flush method

[Techcable]

This is simpler than the API in #332, but requires more work by slog_async to implement it.

See the implementation in slog-rs/async#39

All other slog libraries have a trivial implementation of this, so I will wait to add those those until (and if) this is accepted.

[dpc]

The goal here is to propagate any errors from the inside of the logging?

The interface doesn't say anything about calling it multiple times. Probably good idea to say something, even if "is unspecified".

[Techcable]

I am considering adjusting the flush method to accept a FlushRequest where

enum FlushRequest {
    Block,
    BlockUntil {
        timeout: Option<Duration>,
    },
    NonBlocking,
}

Blocking with a timeout or nonblocking flush only makes sense with slog_async, so I'm not sure if it is worthwhile.

The goal here is to propagate any errors from the inside of the logging?

The goal is to flush buffered streams. Setting up a serde_json::Json with the default settings does not flush output by default, potentially losing messages if the program exits. A similar problem can happen with slog_async, and this is easier to use than an AsyncGuard.

The interface doesn't say anything about calling it multiple times. Probably good idea to say something, even if "is unspecified".

Flushing should be idempotent. I will add this to the docs.