Report progress of streaming base backup.

This commit adds pg_stat_progress_basebackup view that reports
the progress while an application like pg_basebackup is taking
a base backup. This uses the progress reporting infrastructure
added by c16dc1aca5, adding support for streaming base backup.

Bump catversion.

Author: Fujii Masao
Reviewed-by: Kyotaro Horiguchi, Amit Langote, Sergei Kornilov
Discussion: https://postgr.es/m/9ed8b801-8215-1f3d-62d7-65bff53f6e94@oss.nttdata.com

doc/src/sgml/ref/pg_basebackup.sgml

@@ -104,6 +104,13 @@ PostgreSQL documentation
     </listitem>
    </itemizedlist>
   </para>
+
+  <para>
+   Whenever <application>pg_basebackup</application> is taking a base
+   backup, the <structname>pg_stat_progress_basebackup</structname>
+   view will report the progress of the backup.
+   See <xref linkend="basebackup-progress-reporting"/> for details.
+  </para>
  </refsect1>
 
  <refsect1>
@@ -459,6 +466,15 @@ PostgreSQL documentation
         This may make the backup take slightly longer, and in particular it
         will take longer before the first data is sent.
        </para>
+       <para>
+        Whether this is enabled or not, the
+        <structname>pg_stat_progress_basebackup</structname> view
+        report the progress of the backup in the server side. But note
+        that the total amount of data that will be streamed is estimated
+        and reported only when this option is enabled. In other words,
+        <literal>backup_total</literal> column in the view always
+        indicates <literal>0</literal> if this option is disabled.
+       </para>
       </listitem>
      </varlistentry>