Add option to keep furnaces cached in memory

[sgdc3]

Some servers don't mind about the memory usage of keeping all the furnaces loaded at runtime, there should be an option to disable the entire expiring cache system. (Data will be loaded on world load/unload)

[Jikoo]

While I'm not opposed to some system where a BlockProvider handles construction and management of data, I'm unlikely to invest the time myself. The region-esque approach EnchantableBlocks takes isn't uncommon in tracking block data. The reason EB swapped to the current system was that a lot of lag was being caused by too many furnaces being loaded or saved simultaneously.