wip: new merging logic, updated tests

[imorland]

• [x] Delegate merge operations to the queue, where available
• [x] If flarum/lock is available, lock the discussion(s) whilst the merge is pending/in-progress
• [ ] Notify the merge actor when complete
• [x] New merge handler, should overcome previous numbering and constraint related issues
• [x] Tests for most common merge operations

New handler inspired by https://github.com/SychO9/flarum-move-posts