MONGOID-5848 Revert MONGOID-5822 (9.0 backport)

[jamis]

Backport to 9.0

Summary

MONGOID-5822 attempted to fix a regression where child callbacks that depended on parent state were no longer invoked if the child had not changed. However, the fix itself introduced an unacceptable performance regression.

This PR restores the earlier functionality, which will break apps that depend on callbacks being invoked on unmodified children (for example, when a child callback depends simply on the parent having changed state).

For now, the correct way to implement that behavior is to explicitly iterate over the children in a parent callback, e.g.:

class Parent
  include Mongoid::Document
  has_many :children
  after_save { children.each(&:parent_changed_callback) }
end

class Child
  include Mongoid::Document
  belongs_to :parent
  
  def parent_changed_callback
    # ...
  end
end