fix: Use nondurable cursors for pulsar readers (#324)

By default we set subscriptions to use a durable cursor:
https://github.com/streamnative/pulsar-rs/blob/master/PulsarApi.proto#L352

We only run into this when durable is None which can also be avoided
if we use Pulsar::reader() which also sets durable to Some(false).

However, there are cases where users may overwrite the options before
calling `into_reader()`.

src/consumer/builder.rs

@@ -324,6 +324,10 @@ impl<Exe: Executor> ConsumerBuilder<Exe> {
  // would this clone() consume too much memory?
  let (mut config, mut joined_topics) = self.clone().validate::<T>().await?;
 
+ // Internally, the reader interface is implemented as a consumer using an exclusive,
+ // non-durable subscription
+ config.options.durable = Some(false);
+
  // the validate() function defaults sub_type to SubType::Shared,
  // but a reader's subscription is exclusive
  warn!("Subscription Type for a reader is `Exclusive`. Resetting.");