Add check for _id for $set, $unset update operations

[tatu-at-datastax]

Neither $set nor $unset should allow modifying document's primary key (_id field).

Let's add validation for this case.

In addition, work to be done:

1. Refactor handling of update operations out of DocumentUpdater, into separate operation implementations
2. Improve validation of operators (indicate kind of failure, unknown vs unsupported)
3. Allow cross-updater validation ($set and $unset cannot refer to same paths)
4. Add tests for above.