remove concurrent map and use a Hash

It seems it doesn't need to be threadsafe.

Via zendesk#835 (comment)

Author: d1egoaz

lib/kafka/consumer.rb

@@ -79,7 +79,7 @@ def initialize(cluster:, logger:, instrumenter:, group:, fetcher:, offset_manage
       @current_offsets = Hash.new { |h, k| h[k] = {} }
 
       # Map storing subscribed topics with their configuration
-      @subscribed_topics = Concurrent::Map.new
+      @subscribed_topics = Hash.new
 
       # Set storing topics that matched topics in @subscribed_topics
       @matched_topics = Set.new