Make RowIterator more abstract, in that it no longer depends on having a TABLE;
this is because there will be more and more RowIterators that don't logically
belong to a single table. The utility functions that need a TABLE have been
moved into a helper class TableRowIterator that one can inherit from.

Change-Id: I8ad20d14ba9ecb5ae83fb5ef3757a7bfc045bd2a