Ensure threads are running when calling deliver_messages

Currently if the worker thread has died unexpectedly and we subsequently
call `deliver_messages`, the messages won't actually get delivered until
another message is produced. With this change we will restart the worker
thread so it can receive the `deliver_messages` message.

Author: cstyles

lib/kafka/async_producer.rb

@@ -131,6 +131,8 @@ def produce(value, topic:, **options)
     # @see Kafka::Producer#deliver_messages
     # @return [nil]
     def deliver_messages
+      ensure_threads_running!
+
       @queue << [:deliver_messages, nil]
 
       nil