perf: lessen cpu usage in loop for Pub/Sub (#236)

Author: supercaracal

lib/redis_client/cluster/pub_sub.rb

@@ -74,6 +74,8 @@ def next_event(timeout = nil)
             message = pubsub.take_message(timeout)
             return message if message
           end
+
+          sleep 0.001
         end
       end