repmgrd: improve reconnection handling

Previously, if the server being monitored was not available, repmgrd
would always close the existing connection handle and open a new one.

However, in some cases, e.g. a brief network outage, the existing
connection handle is still good and does not need to be reopened.

This could be particularly problematic if monitoring_history is on,
as this risks leaving orphan sessions on the primary which (given
a sufficiently unstable network) could lead to all available backends
being occupied.

Instead, during an outage we now use a new connection to verify
the server is accessible; if the old connection is still available
(e.g. following a short network interruption) we continue using that;
if  not (e.g. the server was restarted), we use the new one.

doc/appendix-release-notes.sgml

@@ -96,6 +96,12 @@
             </para>
           </listitem>
 
+        <listitem>
+            <para>
+              Improve reconnection handling. (GitHub #480).
+            </para>
+          </listitem>
+
         </itemizedlist>
       </para>
     </sect2>