keep messages in the producer until we get the receipt

[Geal]

in case of reconnections, messages previously sent might have been lost, so we should keep them in a queue until we get the send receipt, and if we reconnected, send them again. Before implementing this, we should look at the Java client's implementation and make sure to follow the same semantics

[Geal]

the way it is done right now, we await for the receipt on each message, so there's no queue of messages, only the current message that can be resent after reconnection:
https://github.com/wyyerd/pulsar-rs/blob/1cc9f9a60ea8618c5bc43faff94b22c463aaaa08/src/producer.rs#L461-L495