Check for duplicate note on rename of a notebook

Currently on a rename of a note, zeppelin allows to create multiple notebooks with same name but with different note ids. This is causing couple of issues such as loading the correct notebook based on the id, but updating the wrong notebook based on note name. 

There's couple of inconsistencies on how notebook is referenced for ex. in some places note is referenced by name while other places its referenced by id. So, while renaming two existing notes to same name makes it dangerous to be in the unknown of which note will contain which changes. 

We should check for duplicates before allowing user to rename the note to same name as an existing note. 

It would be nice to add a similar check as create a new note `addOrUpdateNoteNode(note, true);` which doesn't allow user to create a note with same name as an existing note.