Storage engines should not be aware of high level representation of XML field type, that is DOMDocument.

This change is needed to:

• Avoid caching issues like EZP-23939: Cache tracking fails with ContentTypes
• Align for future storage engine changes where field value is meant to simply be (json) serialized

Note: Even if Storage GateWay Handlers will now have to do more work on complex field types like XMLText, this does not have an negative impact of performance as long as SPI Persistence Cache is enabled (introduced in v5.1), which caches the result of the work done there.