Create conflictHandler

On the server-side there should be a conflict handler for letting the right data win.
- [ ] Create an api for building cusom conflictHandlers optional one pr. collection
- [ ] find a way of getting the diff between client and server time - this way client can calc the server time - this would be comparable with timestamps from other clients

Builtin conflictHandlers
- [x] Last write to server wins
- [ ] The newest document wins
- [ ] OT/Merging strategy

Start by resolving the issue's proposed conflict-handler API and the client/server timestamp comparison described in the checklist. Define how per-collection handlers and the listed built-in strategies should behave, including newest-document and OT/merging approaches. No files or tests are named; done means the API and conflict-resolution behavior are specified and implemented with coverage for each supported strategy.

Written by the indexing model from the issue text.