Extend the Rowset object to contain all the capabilities of Rowhash in

Extend the Rowset object to contain all the capabilities of Rowhash in
addition to its legacy capabilities.  Use Rowset to replace Rowhash.
In addition to requiring less code, This removes the 2^32 result row
limitation, uses less memory, and gives better bounds on worst-case
performance.  The Rowhash implementation has yet to be removed. (CVS 6534)

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