Terrain Data Streaming

[sisby-folk]

In order to avoid memory requirements that scale linearly with map area explored, surveyor needs to be able to stream terrain data off of the disk and decompress it as needed by consumers (client sync requests, map mods, etc) instead of loading it all on world start.

This will require a rework of the API and events currently used for mapping.

Requires #23 implicitly, as the total number of loads would massively increase.