Fix pathfinder bugs: returning nil frequently, broken A*, jump through solid nodes...

Fix pathfinder bugs: returning nil frequently, broken A*, jump through solid nodes (#9339)

* Fix pathfinder fail when startpos is over air
* Note down pathfinder restrictions
* Implement real A* search
* Pathfinder: Implement buildPath non-recursively
* Update find_path documentation
* Pathfinder: Check if jump path is unobstructed
* Pathfinder: Fix drop check first checking upwards
* Pathfinder: Return nil if source or dest are solid
* Pathfinder: Use priority queue for open list