"standby switchover": avoid potential race condition with WAL location check

Immediately after the demotion candidate (primary) has shut down, we can't
be absolutely sure that the walreceiver has flushed all WAL to disk, so
checking pg_last_wal_receive_lsn() at that point might not reflect
the actual last available WAL location.

To handle this, we'll loop for a while (timeout controlled by configuration
parameter "wal_receive_check_timeout") before finally deciding whether
the standby is still behind the shut-down primary.

Addresses issue raised in GitHub #518.

doc/appendix-release-notes.sgml

@@ -105,6 +105,15 @@
             </para>
           </listitem>
 
+         <listitem>
+            <para>
+              Add check <link linkend="repmgr-standby-switchover"><command>repmgr standby switchover</command></link>
+              when comparing received WAL on the standby to the primary's shutdown location to avoid a potential
+              race condition if the standby's walreceiver has not yet flushed all received WAL to disk.
+              GitHub #518.
+            </para>
+          </listitem>
+
        </itemizedlist>
       </para>
     </sect2>