Redesigned iterators:

* Removed ugly ReferenceStorageIterator since
  * it stored either a reference or a fake reference
  * you could not use iterator->method
  * you could not assign iterators
* Introduce ContainerWrapperIterator
  * always stores an assignable ContainerWrapper object
  * allows iterator->method()
  * can be used for any other sparse row type

[[Imported from SVN: r1127]]