Disconnect after leaving the group

This commit removes an unnecessary reconnection after
`Kafka::Consumer#stop` is called.
`@cluster.disconnect` closes the connections to all brokers
including the coordinator in spite of the fact that
`@group.leave` requires a connection to the coordinator,
so call `@cluster.disconnect` after `@group.leave`.

Author: abicky

lib/kafka/consumer.rb

@@ -116,7 +116,6 @@ def subscribe(topic_or_regex, default_offset: nil, start_from_beginning: true, m
     def stop
       @running = false
       @fetcher.stop
-      @cluster.disconnect
     end
 
     # Pause processing of a specific topic partition.
@@ -433,6 +432,7 @@ def consumer_loop
       # important that members explicitly tell Kafka when they're leaving.
       make_final_offsets_commit!
       @group.leave rescue nil
+      @cluster.disconnect
       @running = false
       @logger.pop_tags
     end