Actively populate the sector cache on creating a Space object.
Usually happens upon entering a new system from Hyperspace, loading a saved game, starting a new game.
BIG changes due to splitting the StarSystem cache into a separate StarSystemCache class.
Tweak the sector distances used for automatic caching and StarSystemCache survival.
Merge remote-tracking branch 'fluffy/active_sector_cache'

Conflicts:
	src/Space.cpp
	src/Space.h
	src/galaxy/StarSystem.cpp