Persist chunks to disk

Support option to write cached chunks to disk to persist between sessions.

I would like to have all the chunks I download from the server persisted to disk.

This way, all the chunks I flew around collecting are not discarded when I quit and re-open the game to reconnect to the server for whatever reason.

Fair-play

Some servers might have rules about not persisting chunks to disk.

Support a "fair-play" option to encrypt all downloaded server chunk data at rest, and decrypt to load into memory.

Duplicate of #16