StorageUser interface is overly large and should be split

StorageUser is implemented by services which use the Single/Double storage classes in the sakai-kernel-storage-util. The same interface is used across both Single and Double storage even though Single storage only uses less that half of the methods in StorageUser. In classes that implement StorageUser but only use Single Storage there are lots of methods that have to be implemented but just return null. This is confusing for people reading the code.

I'd like to suggest splitting them and removing the methods that are no longer needed.