erl impl: semantic for method `stop_producer`

[alissa-tung]

    the method `stop_producer` will not stop a running producer immediately, it will wait for all request before stop to be handled, then drop NIF appender, then shutdown. if a channel in select is used for this purpose, it will also do graceful shutdown, and it can stop producer from getting more append requests. I am not going to change this behavior in this PR anyway.

Originally posted by @alissa-tung in #73 (comment)

[alissa-tung]

#[rustler::nif]
fn stop_producer(producer: ResourceArc<NifAppender>) -> Atom {
    let producer = &producer.0;
    producer.send(None).unwrap_or(());
    ok()
}

also, the flume::send is blocking, which is a bug

[alissa-tung]

#78
hstreamdb/hstreamdb_erl-rs#17