Make SCRAM iteration count configurable

Replace the hardcoded value with a GUC such that the iteration
count can be raised in order to increase protection against
brute-force attacks.  The hardcoded value for SCRAM iteration
count was defined to be 4096, which is taken from RFC 7677, so
set the default for the GUC to 4096 to match.  In RFC 7677 the
recommendation is at least 15000 iterations but 4096 is listed
as a SHOULD requirement given that it's estimated to yield a
0.5s processing time on a mobile handset of the time of RFC
writing (late 2015).

Raising the iteration count of SCRAM will make stored passwords
more resilient to brute-force attacks at a higher computational
cost during connection establishment.  Lowering the count will
reduce computational overhead during connections at the tradeoff
of reducing strength against brute-force attacks.

There are however platforms where even a modest iteration count
yields a too high computational overhead, with weaker password
encryption schemes chosen as a result.  In these situations,
SCRAM with a very low iteration count still gives benefits over
weaker schemes like md5, so we allow the iteration count to be
set to one at the low end.

The new GUC is intentionally generically named such that it can
be made to support future SCRAM standards should they emerge.
At that point the value can be made into key:value pairs with
an undefined key as a default which will be backwards compatible
with this.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Jonathan S. Katz <jkatz@postgresql.org>
Discussion: https://postgr.es/m/F72E7BC7-189F-4B17-BF47-9735EB72C364@yesql.se

doc/src/sgml/config.sgml

@@ -1132,6 +1132,26 @@ include_dir 'conf.d'
       </listitem>
      </varlistentry>
 
+     <varlistentry>
+      <term><varname>scram_iterations</varname> (<type>integer</type>)
+      <indexterm>
+       <primary><varname>scram_iterations</varname> configuration parameter</primary>
+      </indexterm>
+      </term>
+      <listitem>
+       <para>
+        The number of computational iterations to be performed when encrypting
+        a password using SCRAM-SHA-256. The default is <literal>4096</literal>.
+        A higher number of iterations provides additional protection against
+        brute-force attacks on stored passwords, but makes authentication
+        slower. Changing the value has no effect on existing passwords
+        encrypted with SCRAM-SHA-256 as the iteration count is fixed at the
+        time of encryption. In order to make use of a changed value, a new
+        password must be set.
+       </para>
+      </listitem>
+     </varlistentry>
+
      <varlistentry id="guc-krb-server-keyfile" xreflabel="krb_server_keyfile">
       <term><varname>krb_server_keyfile</varname> (<type>string</type>)
       <indexterm>