Fix the issue that REST API returns unknown after postgres restart (#2956)

Close #2955

patroni/postgresql/connection.py

@@ -147,7 +147,8 @@ class ConnectionPool:
     def close(self) -> None:
         """Close all named connections from Patroni to PostgreSQL registered in the pool."""
         with self._lock:
-            if any(conn.close(True) for conn in self._connections.values()):
+            closed_connections = [conn.close(True) for conn in self._connections.values()]
+            if any(closed_connections):
                 logger.info("closed patroni connections to postgres")