Document how to ensure durable write with event writer

[claudiofahey]

Problem description
It is not clear how to use the event writer to determine when the event has been durably persisted.
Based on logging, writer.write_event(payload).await returns even before the event has been sent across the wire.

Problem location
EventStreamWriter.write_event() and
https://github.com/pravega/pravega-client-rust/blob/master/examples/event_write_and_read.rs#L65

Suggestions for an improvement
The documentation for EventStreamWriter.write_event() should be updated. Also, the example in event_write_and_read.rs should show how to await a durable write ack.

[Tristan1900]

I see. In this code snippet

    let result = event_stream_writer.write_event(payload).await;
    assert!(result.await.is_ok());

the result is a channel that needs to await and when that has returned ok then event is durably persisted.
Will document that to avoid confusion.

[claudiofahey]

This is not a very convenient API as it requires two awaits and then a match statement to flatten the result. It gets even more complicated without the anyhow macro.

let future = writer.write_event_by_routing_key(routing_key, event);
let receiver = future.await;
let result = receiver.await; // Result<Result<(), SegmentWriterError> RecvError>
let flattened_result = match result { // Result<(), Error>
    Ok(r) => r.map_err(|e| anyhow!(e)),
    Err(e) => Err(anyhow!(e)),
};

Is there anyway to get rid of the intermediate future? What is the benefit of it?