Improved fix for ticket [ced41c7c7d6b4d36] that keeps skip-scan enabled,

Improved fix for ticket [ced41c7c7d6b4d36] that keeps skip-scan enabled,
but avoids identifying a skip-scan as order-distinct (because it is not)
and thus forces a separate b-tree to implement the DISTINCT clause of a
query.  This undoes check-in [a871d69c6de65038] and substitutes a new fix.

FossilOrigin-Name: 89bf0399e87be76e7b264e0a35e8a6d3af0f9e71123a929c20754882b636ca70