Performance improvement for sorted merge iterator

Sorted merge iterator has cpu-intensive operations to sort the points
from multiple inputs. Typical queries like `SELECT * FROM m GROUP BY *`
do not behave well due to the comparison of points though in many cases
it doesn't necessarily have to use the slow path.

This patch adds a shortcut.  If each input has a single and unique
series we can just return the points input by input.
The detection of the shortcut introduces slight overhead but the gains
are significant in many slow queries.