FS iterator copy() positioning not quite correct

The core code implements the copy() method for iterators by doing a separate construction of the copy of the iterator, followed by a moveTo(fs) where fs is the feature structure obtained from the original iterator (if it is valid).

The definition of moveTo(fs) though is to move to the left-most of potentially several FSs that compare "equal" using the underlying indexes' compare keys.

This can result in the copy being positioned at a different spot.