Fix last status changed timestamp is not set.

Fix last status changed timestamp is not set.

If there's no status file or -D is specified when starting up
Pgpool-II, the last status changed timestamp was not set and
pcp_node_info command for example did not show the proper timestamp,
rather showed 1970/1/1, which is the Unix timestamp epoch.

Note that correct timestamp is set once clients connect to Pgpool-II
because at that time the status is changed from "waiting" to "up". So
the phenomena is only observed before any client connects to the
server.