Fix possible thread safety issue on non-GVL implementations in subcla…

…sses plugin

lib/sequel/plugins/subclasses.rb

@@ -28,17 +28,25 @@ module ClassMethods
 
         # All descendent classes of this model.
         def descendents
-          subclasses.map{|x| [x] + x.descendents}.flatten
+          Sequel.synchronize{_descendents}
         end
 
         # Add the subclass to this model's current subclasses,
         # and initialize a new subclasses instance variable
         # in the subclass.
         def inherited(subclass)
           super
-          subclasses << subclass
+          Sequel.synchronize{subclasses << subclass}
           subclass.instance_variable_set(:@subclasses, [])
         end
+
+        private
+
+        # Recursive, non-thread safe version of descendents, since
+        # the mutex Sequel uses isn't reentrant.
+        def _descendents
+          subclasses.map{|x| [x] + x.send(:_descendents)}.flatten
+        end
       end
     end
   end