gzip handling serves stale .gz files

[dronus]

If .gz files are found, they should not be served if they are older then the file originally requested. As they are stated anyway, that should not add performance drawbacks, but prevent outdated documents beeing served.

Additionally, a user attached callback could be invoked, which may create a recent .gz file or issue a warning to some log.

Not a bug, merely a small missing feature that may do big headaches.