Remove Mutex in favor of make_true

https://ruby-concurrency.github.io/concurrent-ruby/Concurrent/AtomicBoolean.html#make_true-instance_method

examples/default_worker.rb

@@ -4,6 +4,6 @@ class DefaultWorker
   shoryuken_options queue: 'default', auto_delete: true
 
   def perform(sqs_msg, body)
-    Shoryuken.logger.info("Received message: '#{body}'")
+    Shoryuken.logger.debug("Received message: '#{body}'")
   end
 end

lib/shoryuken/manager.rb

@@ -12,7 +12,7 @@ def initialize(fetcher, polling_strategy)
 
       @done = Concurrent::AtomicBoolean.new(false)
 
-      @dispatch_later_mutex = Mutex.new
+      @dispatch_later = Concurrent::AtomicBoolean.new(false)
 
       @fetcher = fetcher
       @polling_strategy = polling_strategy
@@ -84,13 +84,11 @@ def busy
     end
 
     def dispatch_later
-      return if @_dispatch_later
+      return unless @dispatch_later.make_true
 
-      @dispatch_later_mutex.synchronize do
-        @_dispatch_later ||= after(1) do
-          @_dispatch_later = nil
-          dispatch
-        end
+      after(1) do
+        @dispatch_later.make_false
+        dispatch
       end
     end