Refactored chunk state representation into its own class.

+ added class ChunkState
- removed subtype Chunk.State
* changed class Chunk to use the new class ChunkState
* changed some other classes to use the new class ChunkState

* changed protobuf implementation to simplify encoding/decoding of chunk states

- removed default constructor for class Chunk
- removed implementation of interface Externalizable for class Chunk

Note:
From now on, java object serialization for chunks is no longer supported!