No need to synchronize for just reading an ivar, at least in MRI

Monitor `synchronize` became a little bit more costly since https://github.com/ruby/ruby/commit/f91879a7b5 even when then synchronization is nested,
thus we'd better avoid unnecessary `synchronize {  }` call for the majoriry platform that properly implements GVL.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -118,10 +118,16 @@ def any_waiting?
           end
         end
 
-        # Returns the number of threads currently waiting on this
-        # queue.
-        def num_waiting
-          synchronize do
+        if defined?(JRUBY_VERSION)
+          # Returns the number of threads currently waiting on this queue.
+          def num_waiting
+            synchronize do
+              @num_waiting
+            end
+          end
+        else
+          # Returns the number of threads currently waiting on this queue.
+          def num_waiting
             @num_waiting
           end
         end