[sakai2-tcc] Cleanup of StorageUser

[Matthew Buckett]

I'm proposing to split StorageUser into two interfaces which match the
methods needed by the users of these interfaces in
sakai-kernel-storage-util.

At the moment lots of code classes have methods that return null
because they have to implement the whole of the StorageUser interface
even though more than half of the methods aren't ever called.

Making this change cleans up lots of redundant methods in the code
kernel classes.

https://jira.sakaiproject.org/browse/KNL-899

-- 
  Matthew Buckett
  VLE Developer, LTG, Oxford University Computing Services