Add a conflict mechanism switch off property

The conflict mechanism has been introduced in XWIKI-6665 and might have introduced some unwanted behaviours on custom extensions due to the need of keeping up-to-date the version of the document being edited when saving an object.
This problem is certainly mitigated by XWIKI-175 solution, but we don't have enough feedback right now to be sure.
In order to bulletproof a bit our solution, it would be better to have a switch off property for the conflict detection, to be able to work around any unexpected issue.