Move pg_stat_statements query jumbling to core.

Add compute_query_id GUC to control whether a query identifier should be
computed by the core (off by default).  It's thefore now possible to
disable core queryid computation and use pg_stat_statements with a
different algorithm to compute the query identifier by using a
third-party module.

To ensure that a single source of query identifier can be used and is
well defined, modules that calculate a query identifier should throw an
error if compute_query_id specified to compute a query id and if a query
idenfitier was already calculated.

Discussion: https://postgr.es/m/20210407125726.tkvjdbw76hxnpwfi@nol

Author: Julien Rouhaud

Reviewed-by: Alvaro Herrera, Nitin Jadhav, Zhihong Yu

doc/src/sgml/pgstatstatements.sgml

@@ -20,6 +20,14 @@
   This means that a server restart is needed to add or remove the module.
  </para>
 
+ <para>
+  The module will not track statistics unless query
+  identifiers are calculated.  This can be done by enabling <xref
+  linkend="guc-compute-query-id"/> or using a third-party module that
+  computes its own query identifiers.  Note that all statistics tracked
+  by this module must be reset if the query identifier method is changed.
+ </para>
+
  <para>
    When <filename>pg_stat_statements</filename> is loaded, it tracks
    statistics across all databases of the server.  To access and manipulate
@@ -84,7 +92,7 @@
        <structfield>queryid</structfield> <type>bigint</type>
       </para>
       <para>
-       Internal hash code, computed from the statement's parse tree
+       Hash code to identify identical normalized queries.
       </para></entry>
      </row>
 
@@ -386,6 +394,16 @@
    are compared strictly on the basis of their textual query strings, however.
   </para>
 
+  <note>
+   <para>
+    The following details about constant replacement and
+    <structfield>queryid</structfield> only applies when <xref
+    linkend="guc-compute-query-id"/> is enabled.  If you use an external
+    module instead to compute <structfield>queryid</structfield>, you
+    should refer to its documentation for details.
+   </para>
+  </note>
+
   <para>
    When a constant's value has been ignored for purposes of matching the query
    to other queries, the constant is replaced by a parameter symbol, such