Revert "First serious pass at Oracle support: Oracle supports rownum, not limit."

This reverts commit 77adef1ba608c7b33980acaf3cae79152cdd8725.
Easier to do straight eq comparisons if we are always a string.
Implemented proper Oracle three-nested-select strategy for rownum.

You may find two-nested-select online; ignore them.  We have to support
arbitrarily-constructed aggregate queries, after all.  Better to preserve that
than to attempt to modify it with our additional WHERE conditions.