Add support for forced chunks on the client side

Added support for forced chunks on the client side.

The code is pretty self explanatory, however, there are two downsides to this. Chunks can't be force loaded (they have to be loaded the normal way - by "visiting" the chunk). The other thing is that forced chunks that would normally be unloaded won't receive any updates from the server.

This could be used to export parts of the world to schematic files (part of Schematica).