[CM] Concurrency issues for Visual Editing resulting in totally unpredictable outcome

NodeFieldServiceImpl uses a system session, while the DocumentsServiceImpl uses a user session. That leads to severe concurrency issues with totally unexpected outcomes. ( I have not proven that is does, but it's likely to happen ).

Also the NodeFieldServiceImpl uses the session as a instance variable. NodeFieldServiceImpl is used as a singleton, so it should not have any instance variables.

Passing on the session as argument to each of the three methods solves these issues.