Add calcluation of bitmap storage capacity.

<   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.
>   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.  Frequently
>   accessed bitmaps would have to be stored in shared memory.  One 8k
>   page of bitmaps could track 512MB of heap pages.