The handling of colliding page creation should be improved

Right now, if two instances create documents with the same reference on their side at the same time, one replace the other without any kind of conflict warning (the other is not lost, it's simply moved to the trash bin).

The reason is that this sends creation messages with full history, and this does not apply any kind of merging and conflict handling currently (because it's hard to merge history in a way that make sense).

If any document exists at the location when receiving a creation message, I can think of the following alternatives:
1. behave as now, but trigger the conflict warning so that at least user are aware that something happen and they should check it
2. cheat and handle a received creation message with a single version in the history identical to the current one as an update from empty document so that the same merging/conflict handling is applied as with any other update

I feel the best default would be "2." and we could introduce a preference to switch to "1." if one would prefer it.