fix: thread pool scope and mutex need to be global across all instanc…

…es of Scope for it to be a global thread pool (#471)

lib/graphiti/scope.rb

@@ -3,14 +3,14 @@ class Scope
     attr_accessor :object, :unpaginated_object
     attr_reader :pagination
 
-    @thread_pool_executor_mutex = Mutex.new
+    @@thread_pool_executor_mutex = Mutex.new
 
     def self.thread_pool_executor
       return @thread_pool_executor if @thread_pool_executor
 
       concurrency = Graphiti.config.concurrency_max_threads || 4
-      @thread_pool_executor ||= @thread_pool_executor_mutex.synchronize do
-        Concurrent::ThreadPoolExecutor.new(
+      @@thread_pool_executor_mutex.synchronize do
+        @@thread_pool_executor ||= Concurrent::ThreadPoolExecutor.new(
           min_threads: 0,
           max_threads: concurrency,
           max_queue: concurrency * 4,