Moved Bukkit's keepSpawnInMemory logic to DimensionManager. Fixes #463

Back in 1.6 beta, CraftBukkit added the following patch
https://github.com/Bukkit/CraftBukkit/commit/9ba9643b89d46bedb950c928796420b8491368e5
to do an initial chunk load for all dimensions by default unless keepSpawnInMemory was in false.
Not only did this cause issues with certain Forge mods but it also
decreased server startup time depending on the amount of dimensions you
had registered at startup. Now only overworld will do an initial chunk
load for compatibility with forge.