pgsql: Put back planner's ability to cache the results of

Log Message:
-----------
Put back planner's ability to cache the results of mergejoinscansel(),
which I had removed in the first cut of the EquivalenceClass rewrite to
simplify that patch a little.  But it's still important --- in a four-way
join problem mergejoinscansel() was eating about 40% of the planning time
according to gprof.  Also, improve the EquivalenceClass code to re-use
join RestrictInfos rather than generating fresh ones for each join
considered.  This saves some memory space but more importantly improves
the effectiveness of caching planning info in RestrictInfos.

Modified Files:
--------------
    pgsql/src/backend/nodes:
        copyfuncs.c (r1.362 -> r1.363)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/copyfuncs.c.diff?r1=1.362&r2=1.363)
        outfuncs.c (r1.294 -> r1.295)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/nodes/outfuncs.c.diff?r1=1.294&r2=1.295)
    pgsql/src/backend/optimizer/path:
        costsize.c (r1.176 -> r1.177)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/costsize.c.diff?r1=1.176&r2=1.177)
        equivclass.c (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/path/equivclass.c.diff?r1=1.1&r2=1.2)
    pgsql/src/backend/optimizer/prep:
        prepunion.c (r1.136 -> r1.137)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/prep/prepunion.c.diff?r1=1.136&r2=1.137)
    pgsql/src/backend/optimizer/util:
        restrictinfo.c (r1.52 -> r1.53)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/optimizer/util/restrictinfo.c.diff?r1=1.52&r2=1.53)
    pgsql/src/backend/utils/adt:
        selfuncs.c (r1.220 -> r1.221)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/selfuncs.c.diff?r1=1.220&r2=1.221)
    pgsql/src/include/nodes:
        relation.h (r1.133 -> r1.134)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/nodes/relation.h.diff?r1=1.133&r2=1.134)
    pgsql/src/include/utils:
        selfuncs.h (r1.38 -> r1.39)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/selfuncs.h.diff?r1=1.38&r2=1.39)