Grav ship cache

[Sakura-TA]

This patch is to cache the gravship search result wisely.
default in vanilla, a redo of search is happening when one of the following situation is matched:

1. cached gravship is not in map currently searching
2. cached tick is not current tick

Means in the following situation it always do the search and iter through all the items:

1. player don't have a ship in the map currently searching.
2. player have a ship in this map but the search haven't been done in this tick yet.
3. player have a ship in this map and cached but another ship in other map recached and took the cache slot so a mis-cache happens.

This patch use cache to prevent all-item-iter happens in these 3 situation.

1. Only allowed 1 call per-map-per-tick if there's no gravship.(instead of do the search for every call in same tick)
2. Check if gravship previously cached is still available and reuse it.(instead of do the search every tick)
3. Fix by using per-map-cache.

It's working fine to me for now and fixed the problems I mentioned here, but I believe further works is still available on it, like "check the gravship's ownership" or "make 2 ships on same map clearly independent", So I'll draft this PR here for a little while before I'm sure with what should be added here, and also for a longer test.