Local voxel cache & some bug fixes

- supports a local cache per domain
- shows voxels from the cache even before domain servers respond with any content, makes switching and starting domains seem really fast.
- Note: does not yet support versioning/last modified to reduce server to client traffic

Also includes this bug fix:
- octree servers (particles and voxels) were supposed to suppress duplicate packets, that code was broken recently due to a typo, now it's fixed again.