Map of nested discriminators

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

**What is the current behavior?**
It would be very useful to allow for a map of nested discriminators.
While you could make a case that an array of embedded discriminators is a better schema design, sometimes you have to work with a predetermined data structure.

**What is the expected behavior?**
```
const shapeSchema = Schema({ name: String }, { discriminatorKey: 'kind' });
const schema = Schema({ shape: {type: Map, of: shapeSchema} });

// error
schema.path('shape').discriminator('Circle', Schema({ radius: String }));
schema.path('shape').discriminator('Square', Schema({ side: Number }));

const MyModel = mongoose.model('ShapeTest', schema);

let doc = new MyModel({ shape: {a: { kind: 'Circle', radius: 5 }, b: { kind: 'Square', side: 10 }} });
doc.shape.a.radius; // 5
doc.shape.b.side; // 10
```

**What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.**

Node.js 10.16.3
Mongoose 5.7.12
MongoDB 4.0.3

Start by reproducing the schema and document example from the issue with the stated Node.js, Mongoose, and MongoDB versions. Then trace how Map paths and discriminators are handled, comparing the requested nested behavior with embedded discriminators. Done means Circle and Square entries in separate map keys hydrate with their respective fields without the shown error.

Written by the indexing model from the issue text.