Made ezroot cluster aware

> Part of [EZP-22978](http://jira.ez.no/browse/EZP-22978)

The PR does two things

### Adds an `applyServerUri` method to cluster handlers/interface
This gives cluster handlers an opportunity to change an URI so that the file is linked to its actual location. Existing handlers (FS/DFS) won't apply any transformation, while handlers such as DFS with S3 will.

### Makes `eZURI::transformUri()` call `applyServerUri()`
Makes `eZURI::transformUri()` use the newly added cluster `applyServerUri()` method (added in b8eb4ce) when called from `ezroot` (`$ignoreIndexDir == true`).

I guess there's a missing `htmlspecialchars` call here to have the same behavior as the original implementation