pgsql: Add idea on auto-cluster: < store heap rows in hashed groups.

Log Message:
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Add idea on auto-cluster:

< store heap rows in hashed groups.

store heap rows in hashed groups, perhaps using a user-supplied
hash function.

Modified Files:
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pgsql/doc:
TODO (r1.1373 -> r1.1374)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/TODO.diff?r1=1.1373&amp;r2=1.1374)