This provides information about the numbers of tuples that were visited
but not returned by table scans, as well as the numbers of join tuples
that were considered and discarded within a join plan node.

There is still some discussion going on about the best way to report counts
for outer-join situations, but I think most of what's in the patch would
not change if we revise that, so I'm going to go ahead and commit it as-is.

Documentation changes to follow (they weren't in the submitted patch
either).

Marko Tiikkaja, reviewed by Marc Cousin, somewhat revised by Tom