Another MongoDB dialect's idiosyncrasy: `_id` must be the primary key field name.

So for the following example Entity class:

@Entity
class Book {
    @Id
    int id;
}

The `id` entity field should be mapped and persisted in `_id` primary key field name in MongoDB.

But if primary key column name has been specified as below example:

@Entity
class Book {
    @Id
    @Column(name = "id")
    int id;
} 

We have to violate user's intention by throwing exception to explain that MongoDB has its hardcoded field name.

Note: the above second requirement has been excluded from this ticket and in the scope of its own ticket at https://jira.mongodb.org/browse/HIBERNATE-56