Avoid class_attrs for unused collection callbacks

Previously we would generate 4 class attributes for each collection
association whether or not they had any {before,after}_{add,remove}
callbacks.

Class attributes are relatively expensive as they eval code which then
redefines methods on assignment.

I don't think these attributes were intended to be a public API (docs do
not reference them, and we don't allow reconfiguring other callbacks
after associations have been defined, so I think these should be safe to
skip when empty.

activerecord/lib/active_record/associations/builder/collection_association.rb

@@ -30,11 +30,18 @@ module ActiveRecord::Associations::Builder # :nodoc:
     def self.define_callback(model, callback_name, name, options)
       full_callback_name = "#{callback_name}_for_#{name}"
 
-      unless model.method_defined?(full_callback_name)
+      callback_values = Array(options[callback_name.to_sym])
+      method_defined = model.respond_to?(full_callback_name)
+
+      # If there are no callbacks, we must also check if a superclass had
+      # previously defined this association
+      return if callback_values.empty? && !method_defined
+
+      unless method_defined
         model.class_attribute(full_callback_name, instance_accessor: false, instance_predicate: false)
       end
 
-      callbacks = Array(options[callback_name.to_sym]).map do |callback|
+      callbacks = callback_values.map do |callback|
         case callback
         when Symbol
           ->(method, owner, record) { owner.send(callback, record) }

activerecord/lib/active_record/associations/collection_association.rb

@@ -480,7 +480,11 @@ module ActiveRecord
 
         def callbacks_for(callback_name)
           full_callback_name = "#{callback_name}_for_#{reflection.name}"
-          owner.class.send(full_callback_name)
+          if owner.class.respond_to?(full_callback_name)
+            owner.class.send(full_callback_name)
+          else
+            []
+          end
         end
 
         def include_in_memory?(record)