EZP-21775: Make eZFSFileHandler implement eZClusterFileHandlerInterface

At the moment, `eZFSFileHandler` doesn't implement the `eZClusterFileHandler` interface which can cause problems when we use `eZClusterFileHandler::instance()`, get an `eZFSFileHandler` as a result and then use methods which are not implemented.

https://jira.ez.no/browse/EZP-21775

This PR makes `eZFSFileHandler` implement `eZClusterFileHandler`.

The method `isFileExpired` had to be changed to non-static, but will not break when called statically. (In general, IMO, the method *should* be static, but this is not in the scope of this PR.)

I made the new methods call `isFileExpired` statically, because the same is done in `eZDFSFileHandler`.

Cheers
:octocat: Jérôme