Improved query optimization for multi-column indexes where the second or

Improved query optimization for multi-column indexes where the second or
later columns are constrained by an IN operator and the earlier index columns
limit the search to a small number of rows.  Use the new OP_SeekScan opcode
which does scanning of the relevant range of the index but gives up and
falls back to doing a seek if the number of rows scanned grows to large,
in order to guard against pathological cases where the estimated number
of rows to be scanned is far too small.

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