Count issue with related models using though model

@regevbr がすでに取り組んでいます。

2020年1月21日 から。

## Steps to reproduce

Create a model called Book with the properties id,author,title
Add a hasMany relation between the default User model and the Book model using a through Model called UserToBook which has the following properties: userId,bookId

The relation is defined in the User model like so.
```json
"books": {
"type": "hasMany",
"model": "Book",
"foreignKey": "userId",
"through": "UserToBook",
"keyThrough": "bookId"
},
```

## Current Behavior

Querying the user for his books using a where filter works as expected (there is a bug there as well which I will address in a different issue). The reason it works is the `related` function
https://github.com/strongloop/loopback-datasource-juggler/blob/d54d76947724d75a0f46b151c064742c678b0bbe/lib/scope.js#L61
Which, after querying the through model, also queries the related model directly.

When trying to count the number of books of a user, using a where filter (on the author for example), the result is the entire number of books the user has and not the number of books from that author. The reason for that is that the count function
https://github.com/strongloop/loopback-datasource-juggler/blob/d54d76947724d75a0f46b151c064742c678b0bbe/lib/scope.js#L509-L528
Only queries the though model, and not the related model directly.

## Expected Behavior

Return the actual count, taking the filter under account

## Link to reproduction sandbox

Working on it

## Additional information

It seems that the findOne,updateAll and destroyAll suffer from the same issue