pgsql: Add calcluation of bitmap storage capacity.

Log Message:
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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.

Modified Files:
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pgsql/doc:
TODO (r1.1709 -> r1.1710)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1709&r2=1.1710)
pgsql/doc/src/FAQ:
TODO.html (r1.216 -> r1.217)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.216&r2=1.217)