Follow https://github.com/mongodb/laravel-mongodb/issues/2626

In PHPORM-50, the implementation of Builder::count() was modified to not use the deprecated Collection::count(), but the new Collection::countDocuments(). This have a negative impact on performance when people needs to count all documents of a collection.

Proposition: Add a new method Builder::estimatedCount() that it can also be used from the model: Model::estimatedCount().

We need to throw an exception if this method is used while the query builder have $wheres predicated, because that's not supported by MongoDB Server.