Changelog for #2707/#2722
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.
Move GenSectorCache to SectorCache and make SectorCache internals private.
Use only sector part of SystemPath for SectorCache.
Get rid of copying in SectorCache (use RefCountedPtr and unique_ptr).
Improve faction handling with ref counted sector cache.
Split SectorCache in refcounted slave caches and non-refcounted master cache.
Only the SectorCache may generate Sectors now.
Multithreading fixes for SectorCacheJobs.