fix: Implement `flush` function on `Async`

[gabyx]

See https://github.com/slog-rs/slog/pull/332

[gabyx]

Not really sure if that feature is needed: The use case was #35 :

• Make the worker thread somehow stop processing records (not implemented)
• Flush all pending stuff (till the crossbeam channels are empty) implemented
• Output some prompt on letssay stderr.
• Make the worker thread continue to process records.

So if we would have a term logger we could before prompting just use log.flush(...) and then show the prompt and wait for input etc...

[dpc]

Looks reasonable to me. Ping @Techcable as current maintainer.