Revisiting the IN-scan optimization to try to fix it for the corner case

Revisiting the IN-scan optimization to try to fix it for the corner case
where the statistics deceive the query planner into using a scan when
an indexed lookup would be better.  This check-in changes the code
generation to do the IN-scan using a new OP_SeekScan opcode.  That new
opcode is designed to abandon the scan and fall back to a seek if
it doesn't find a match quickly enough.  For this work-in-progress check-in,
OP_SeekScan is still a no-op and OP_SeekGE still ends up doing all the work.

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