We want to use the existing `IMongoQueryable` in LINQ3, but the existing interface defines a method called `GetExecutionModel` that returns a `QueryableExecutionModel`.

The closest class in LINQ3 is `ExecutableQuery`. Fortunately `QueryableExecutionModel` is an abstract class, so we can subclass it.

Add a class called `QueryableExecutionModel3` that wraps an instance of `ExecutableQuery` and implements the abstract method defined in the abstract base class.