KafkaIdempotentRepository cache incorrectly flagged as ready

The `KafkaIdempotentRepository` initialises its cache off the back of the pre-existing Kafka topic with previous entries, with the following code:

log.debug("Subscribing consumer to {}", topic);             
consumer.subscribe(Collections.singleton(topic));             
log.debug("Seeking to beginning");             
consumer.seekToBeginning(consumer.assignment());
             
POLL_LOOP: while (running.get()) {                 
  log.trace("Polling");                 
  ConsumerRecords<String, String> consumerRecords = consumer.poll(pollDurationMs);                 
  if (consumerRecords.isEmpty()) {                     
    // the first time this happens, we can assume that we have consumed all messages up to this point                     
    log.trace("0 messages fetched on poll");                     
    if (cacheReadyLatch.getCount() > 0) {                         
      log.debug("Cache warmed up");                         
      cacheReadyLatch.countDown();                     
    }
  }

The problem with this code is:

1. `consumer.subscribe` doesn't instantaneously assign partitions to the consumer
2. When `consumer.seekToBeginning` is called, the operation doesn't do anything because it has no partitions yet (see seekToBeginning doesn't work without auto.offset.reset (apache.org) 
3. When later the first `consumer.poll` is issued, it returns nothing, triggering the sequence to confirm the cache as ready when it isn't yet. That can cause upstream messages not been correctly de-duplicated.  

The solution is:

1. Use a different overload of `consumer.subscribe` that accepts an implementation of the `ConsumerRebalanceListener`.
2. When partitions are assigned to the `consumer` instance, call `seekToBeginning` there.
3. Doing an initial `poll(0)` that will never return records but will force the partition assignment process