Revert patch for taking fewer snapshots.

This reverts commit d573e239f0, "Take fewer
snapshots".  While that seemed like a good idea at the time, it caused
execution to use a snapshot that had been acquired before locking any of
the tables mentioned in the query.  This created user-visible anomalies
that were not present in any prior release of Postgres, as reported by
Tomas Vondra.  While this whole area could do with a redesign (since there
are related cases that have anomalies anyway), it doesn't seem likely that
any future patch would be reasonably back-patchable; and we don't want 9.2
to exhibit a behavior that's subtly unlike either past or future releases.
Hence, revert to prior code while we rethink the problem.

doc/src/sgml/release-9.2.sgml

@@ -809,13 +809,6 @@
        </para>
       </listitem>
 
-      <listitem>
-       <para>
-        Take fewer <acronym>MVCC</acronym> snapshots
-        (Robert Haas)
-       </para>
-      </listitem>
-
       <listitem>
        <para>
         Make the number of CLOG buffers scale based on <link