Unified Sector and StarSystem caches

Contains the commits of #2879, the first commit unique to this PR is "Extract templated base class from SectorCache." (currently b876620, but may change on history rewriting).

As mentioned in the synchronous jobs PR, this unifies Sector and StarSystem caches by creating a templated base class for the caches. The ```StarSystem``` cache is filled with synchronous jobs when a new ```Space``` is created and once the ```Sector``` cache is successfully filled.