Fix cookie session timestamp validation

[loocars]

In certain scenarios localhost could be blocked or take a long time to
resolve, hence the cookie session validation now uses the loopback
address directly instead of localhost

[LarsMichelsen]

While this may help in certain scenarios, there are others which will break. There are IPv6 only systems out there which would not understand 127.0.0.1.

Ideas I have to solve it differently: We already use SERVER_PORT and may be able to use SERVER_HOST in addition.
Alternatively, we could also try to get the OMD config options (APACHE_TCP_ADDR, APACHE_TCP_PORT. They might be available as environment variable CONFIG_APACHE_TCP_PORT and CONFIG_APACHE_TCP_ADDR.