pgsql: Improve handling of prune/no-prune decisions by storing a page's

Log Message:
-----------
Improve handling of prune/no-prune decisions by storing a page's oldest
unpruned XMAX in its header.  At the cost of 4 bytes per page, this keeps us
from performing heap_page_prune when there's no chance of pruning anything.
Seems to be necessary per Heikki's preliminary performance testing.

Modified Files:
--------------
    pgsql/contrib/pageinspect:
        README.pageinspect (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/README.pageinspect?r1=1.2&r2=1.3)
        pageinspect.sql.in (r1.2 -> r1.3)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/pageinspect.sql.in?r1=1.2&r2=1.3)
        rawpage.c (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/pageinspect/rawpage.c?r1=1.1&r2=1.2)
    pgsql/doc/src/sgml:
        storage.sgml (r1.18 -> r1.19)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?r1=1.18&r2=1.19)
    pgsql/src/backend/access/heap:
        README.HOT (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/README.HOT?r1=1.1&r2=1.2)
        heapam.c (r1.241 -> r1.242)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c?r1=1.241&r2=1.242)
        pruneheap.c (r1.1 -> r1.2)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/pruneheap.c?r1=1.1&r2=1.2)
    pgsql/src/backend/storage/page:
        bufpage.c (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/page/bufpage.c?r1=1.74&r2=1.75)
    pgsql/src/include/catalog:
        catversion.h (r1.426 -> r1.427)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.426&r2=1.427)
    pgsql/src/include/storage:
        bufpage.h (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/bufpage.h?r1=1.74&r2=1.75)