Refactor|World|SectorCluster: Extracted Subsector and simplified SectorCluster on server side

SectorCluster no longer observes movement of planes and/or lighting
changes on server side.

Subsector was moved to new source files and is now only used by the
client (SectorCluster uses ConvexSubspace directly, on server side).

Began moving client only elements of ConvexSubspace to Subsector...