Implement chunk save/load

[harrynull]

Untested
Do not merge.

[DWVoid]

@harrynull only one question. Does it work properly or is this just a procedural practice?

[DWVoid]

And please consider async IO API as we are dealing with protentially tens of megabytes of data per second and it is not a good idea to jam the worker thread for this

[harrynull]

It's totally untested as the game was crashing at the time I implemented it.
As for the Async IO, I thought that the workers for read-only tasks are automatically multithreaded...

[DWVoid]

@harrynull Yes, they are multi-threaded, but the implementation does not take system calls into account, so it will block the execution thread. There is no good cross-platform solution for this, so better do it explicitly.
I did notice currently the game might crash upon startup, but I just cannot reproduce it often enough with the debugger attached so that the problem can be located. Maybe it could just be a timing issue, or it might be a problem with the network 'GetChunk' protocol