Farming update

Updates seeds into comestibles, adds a minimum and maximum time for them to grow and randomizes growing, and cleans up some code using furniture.load_id in grow_plants in map.cpp.

Edit: I do still need to readd the old seeds for backwards compatibility and redo the item spawns, I realized.
There's a reason this was being called in a loop before.
If you leave and return, the plant will only ever grow by one level, no matter how long you were gone.
In other words, if a seed is more than plantEpochMax * 2 days old when processed, it should grow by at least two steps, but with this code it will not do so.
Also it looks like you can abuse this by leaving and re-entering the area repeatedly to re-roll your chance of plants growing.