Change the entity id type to long and simplify the entity handling

Changes:
* Change the entity id type from int to long.
* Don't reuse entity ids 
      * => We don't need to maintain and store a list of free enttiy ids anymore
      * => We don't need to maintain and store the external references of chunks and players

=> The main purpose of this pull request is simplification in order to make it easier to implement an autosaving that doesn't interrupt the game with a short freeze.