Performance: avoid cloning when saving document if `minimize: false`

[vkarpov15]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

In $__handleSave() we have the following:

  if (this.$isNew) {
    // send entire doc
    const obj = this.toObject(saveToObjectOptions);

However, if minimize: false, we don't need to run toObject(), we can just do const obj = this._doc because the MongoDB driver will clone and apply toBSON() anyway. This should significantly help performance.

Re: #8024, let's implement this functionality first and make sure it works as intended before considering making minimize default to false.