pgsql: Add idea: < cmin/cmax pair and is stored in local memory.

Log Message:
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Add idea:

< cmin/cmax pair and is stored in local memory.

cmin/cmax pair and is stored in local memory. Another idea is to
store both cmin and cmax only in local memory.

Modified Files:
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pgsql/doc:
TODO (r1.1653 -> r1.1654)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1653&amp;r2=1.1654)
pgsql/doc/src/FAQ:
TODO.html (r1.160 -> r1.161)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html.diff?r1=1.160&amp;r2=1.161)