Avoid acknowledge last message if the channel is already closed

[alexcams]

When the stop function is called to finish consumer thread and cleanup rabbitmq connection, a signal is sent to the queue (@internal_queue.put(INTERNAL_QUEUE_POISON)) to finish thread handler, then channel and connection are closed right after sending the signal. There's the possibility that the poll executes a cycle just before consume the exit signal but with the channel already closed where [last message is acknowledge] (https://github.com/logstash-plugins/logstash-integration-rabbitmq/blob/main/lib/logstash/inputs/rabbitmq.rb#L264), raising this exception

We'd need to check if channel is closed before sending this ack, or handle the exception properly and warn that last message hasn't been acknowledged.