Proposal: wasModified() check

**Do you want to request a *feature* or report a *bug*?**
Feature

**What is the current behavior?**
There is no ability to detect if a property _was_ modified after a `save` operation, except by manual tracking of properties, the thought of which makes you 🤢.

**Use case**
I need to update denormalized data across several collections, _AFTER_ my document is saved. I don't want to denormalize _BEFORE_ the document gets saved.

So I want to make a post save hook on the model like so:

```js
/**
* Update denormalized data
*/
ProjectSchema.post('save', async function(next) {

//Check if name modified
if (!this.isModified('name')) {
return next();
}

//Make patch
const {_id, name} = this;
const patch = {name};
const models = ['Contract'];

//Update models
await Promise
.map(models, model => mongoose
.model(model)
.updateDenormalized('project', _id, patch));

//Onwards
next();
});
```

The problem is, that since this code runs _after_ the save, the `isModified` flag will come back as false, always. So there is no way to detect in a post-save hook if a property was modified in the preceding save operation, unless I use _another_ pre-save hook and a bunch of helper properties to track modified state.

A `wasModified` check could solve this by checking and storing which properties have been modified just before a save operation, and making this available for querying through a `wasModified(key)` method.

If the document fails to save, the method will return false.

Thoughts? I can look at submitting a PR if you think this would make sense.

Start by tracing Mongoose document save middleware, especially the post('save') and pre-save hook behavior described in the issue, along with the existing isModified check. Define how modified paths should be captured before saving, exposed through wasModified(key), and reset when a save fails. Done means a post-save hook can reliably detect changes from the preceding save without manual tracking.

Written by the indexing model from the issue text.