Simplified handling of use_debug_cursor.

Turned it from a tri-valued boolean into a regular boolean.
Fixed #20420 -- Normalized query counts on Oracle.

This is achieved by inserting a fake entry in connection.queries when
not releasing a savepoint (since Oracle doesn't support that operation.)

Also removed the can_release_savepoints feature that was recently added,
but is superseded by this solution.
Fixed #17552 -- Removed a hack for IE6 and earlier.

Thanks Aaron Cannon for the report and Tim Graham for the review.