Fix memory leak in shutdown of NetworkSystemImpl.

It was not properly clearing the field storageManager, which in turn was
still referencing the entity manager.
Fix a serve memory leak by properly disposing the liquid wrapper.

The liquid wrapper references indirectly the entity manager.
So not disposing it caused a big memory leak.
Don't announce the chunk provider disposal, when there are still threads creating chunks.

Otherwise the chunk monitor can end up with references to chunks,
after the chunk provider disposal is done.
Merge pull request #1285 from flo/memory-leak-fixes

Memory leak fixes