Improve `partition_load_balance` (#2206)

Spin-off from #2005. Make the primary load balancing cleaner and faster
and more maintainable.
Thus:
- remove all MPI calls, this is now purely local
- remove temporary data structures and/or coral them into their own
little scopes
- simplify `super_cells` vs `regular_cells`
- sort less
- sparse connection tables

Partially inspired by external feedback