Use a timer to try reconnecting to the XMPP server only each 2 seconds

When the connection is lost, immediately try to reconnect, then try to
reconnect every 2 seconds.  This is much better than the previous “Try to
re-connect as fast as possible”.

src/xmpp/xmpp_component.hpp

@@ -241,6 +241,12 @@ public:
    * Whether or not we ever succeeded our authentication to the XMPP server
    */
   bool ever_auth;
+  /**
+   * Whether or not this is the first consecutive try on connecting to the
+   * XMPP server.  We use this to delay the connection attempt for a few
+   * seconds, if it is not the first try.
+   */
+  bool first_connection_try;
 
 private:
   /**