Per-dimension day/night cycles

Custom dimensions should have customizable day/night cycles instead of sharing the same cycle with the Overworld and all other dimensions. This means that:

• Each dimension should have an independent time property which can be modified (with /time) without affecting other dimensions.
• There should be a field in dimension_type/<dimension name> which allows for changing the length of the day, so if you wanted your dimension to "rotate on its axis" twice as fast as the Overworld you could add "day_length": 12000 to said file.
• The gamerule doDaylightCylce should be removed and its functionality moved to /time, so eg. /time set_passage false prevents time from advancing in that particular dimension.