Fix elusive 5 year old bug that caused mgvalleys large caves to be flat and
limited to mapchunk borders.
Error was fixed 2 years ago in 'CavesV6' but not in 'CavesRandomWalk'.
* Rename IWritableNodeDefManager to NodeDefManager
* Make INodeDefManager functions const
* Use "const *NodeDefManager" instead of "*INodeDefManager"
* Remove unused INodeDefManager class
* Merge NodeDefManager and CNodeDefManager
* Document NodeDefManager