Allow choosing between several conflict resolutions

We need to configure and maybe a component that can be extended and in charge of choosing what to so in case of conflicting messages.

Some resolutions ideas:

• two version + merge: current behavior
• two versions with no merge: could be interesting for conflict related to new document
• two documents: could be interesting for new document conflict, but dealing with the rename is actually quite complex (for example there could be subsequent modification of the renamed document coming from the same but with the wrong name)