Deduce equality constraints that are implied by transitivity of

Deduce equality constraints that are implied by transitivity of
mergejoinable qual clauses, and add them to the query quals.  For
example, WHERE a = b AND b = c will cause us to add AND a = c.
This is necessary to ensure that it's safe to use these variables
as interchangeable sort keys, which is something 7.0 knows how to do.
Should provide a useful improvement in planning ability, too.