Simplify unification, rocks_close/1, and cleanup

- Move dbref cleanup code to destructor
- Fix rocks_close/1 cleanup
- Add acquire_rocks_ref() for consistent handling of dbref::symbol
- Simplify error handling cleanup by using unique_ptr
- Fix some potential memory/resource leaks
Dbref blob save/load gives an error
Added some "consts"; changed some pointers to references
Added rocks_alias_lookup/2