pgsql: Move the tuple freezing point in CLUSTER to a point further back

Log Message:
-----------
Move the tuple freezing point in CLUSTER to a point further back in the past,
to avoid losing useful Xid information in not-so-old tuples.  This makes
CLUSTER behave the same as VACUUM as far a tuple-freezing behavior goes
(though CLUSTER does not yet advance the table's relfrozenxid).

While at it, move the actual freezing operation in rewriteheap.c to a more
appropriate place, and document it thoroughly.  This part of the patch from
Tom Lane.

Modified Files:
--------------
    pgsql/src/backend/access/heap:
        rewriteheap.c (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/rewriteheap.c.diff?r1=1.4&r2=1.5)
    pgsql/src/backend/commands:
        cluster.c (r1.159 -> r1.160)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/cluster.c.diff?r1=1.159&r2=1.160)
        vacuum.c (r1.350 -> r1.351)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuum.c.diff?r1=1.350&r2=1.351)
        vacuumlazy.c (r1.88 -> r1.89)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/vacuumlazy.c.diff?r1=1.88&r2=1.89)
    pgsql/src/include/access:
        rewriteheap.h (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/rewriteheap.h.diff?r1=1.1&r2=1.2)
    pgsql/src/include/commands:
        vacuum.h (r1.70 -> r1.71)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/commands/vacuum.h.diff?r1=1.70&r2=1.71)