Made Legacy IO handler server URI aware

> Ready for review
> [EZP-23111](http://jira.ez.no/browse/EZP-23111)

Makes the new stack compatible with the addition of the `applyServerUri()` method recently [applied to the cluster interfaces](https://github.com/ezsystems/ezpublish-legacy/commit/f1392b4cf3db8c676db29e2868fa5d79c07850a9), making it possible to use absolute URIs on a different host for images.

Tested manually...