pgsql: Add all heap page rows visible bitmap idea: < the heap.

Log Message:
-----------
Add all heap page rows visible bitmap idea:

<   the heap.  One way to allow this is to set a bit to index tuples
>   the heap.  One way to allow this is to set a bit on index tuples
<   be cleared when a heap tuple is expired.
<
>   be cleared when a heap tuple is expired.  Another idea is to maintain
>   a bitmap of heap pages where all rows are visible to all backends,
>   and allow index lookups to reference that bitmap to avoid heap
>   lookups, perhaps the same bitmap we might add someday to determine
>   which heap pages need vacuuming.

Modified Files:
--------------
    pgsql/doc:
        TODO (r1.1708 -> r1.1709)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1708&r2=1.1709)
    pgsql/doc/src/FAQ:
        TODO.html (r1.215 -> r1.216)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.215&r2=1.216)