When the query planner has the opportunity to use an IN operater constraint

When the query planner has the opportunity to use an IN operater constraint
on a term of an index other than the left-most term, use the estimated number
of elements on the right-hand side of the IN operator to determine if makes
sense to use the IN operator with index looks, or to just do a scan over the
range of the table identified by the index terms to the left.   Only do this
if sqlite_stat1 measurements are available as otherwise the performance
estimates will not be accurate enough to discern the best plan.  Bias the
decision slightly in favor of using index lookups on each element of the IN
operator.

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