Turn prepared statements back on

We once turned these off (in commit abe71a767b266a157836c0fd45330ec415720bef) because they caused the PostgreSQL query optimizer to use very suboptimal plans. However, PostgreSQL 9.2 has supposedly fixed this:

http://www.postgresql.org/docs/9.2/static/release-9-2.html

So let's try again.

Created by  Eelco Dolstra  on February 25, 2013
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