KAFKA-19820 remove the unnecessary copy from AbstractFetch#fetchablePartitions (#20745)

Remove the redundant copy and change the return type to List.  This
change also align `ShareConsumer`

Reviewers: Kirk True <[email protected]>, Ken Huang
 <[email protected]>, Chia-Ping Tsai <[email protected]>

Author: TaiJuWu

clients/src/main/java/org/apache/kafka/clients/consumer/internals/AbstractFetch.java

@@ -341,15 +341,15 @@ protected FetchRequest.Builder createFetchRequest(final Node fetchTarget,
      * until the previously-fetched data has been processed.
      *
      * @param buffered The set of partitions we have in our buffer
-     * @return {@link Set} of {@link TopicPartition topic partitions} for which we should fetch data
+     * @return {@link List} of {@link TopicPartition topic partitions} for which we should fetch data
      */
-    private Set<TopicPartition> fetchablePartitions(Set<TopicPartition> buffered) {
+    private List<TopicPartition> fetchablePartitions(Set<TopicPartition> buffered) {
         // This is the test that returns true if the partition is *not* buffered
         Predicate<TopicPartition> isNotBuffered = tp -> !buffered.contains(tp);
 
         // Return all partitions that are in an otherwise fetchable state *and* for which we don't already have some
         // messages sitting in our buffer.
-        return new HashSet<>(subscriptions.fetchablePartitions(isNotBuffered));
+        return subscriptions.fetchablePartitions(isNotBuffered);
     }
 
     /**
@@ -430,7 +430,7 @@ protected Map<Node, FetchSessionHandler.FetchRequestData> prepareFetchRequests()
         Set<TopicPartition> buffered = Collections.unmodifiableSet(fetchBuffer.bufferedPartitions());
 
         // This is the set of partitions that do not have buffered data
-        Set<TopicPartition> unbuffered = fetchablePartitions(buffered);
+        List<TopicPartition> unbuffered = fetchablePartitions(buffered);
 
         if (unbuffered.isEmpty()) {
             // If there are no partitions that don't already have data locally buffered, there's no need to issue