Added isForceLoadingChunks flag to ForgeChunkManager to prevent loadChunk

getting stuck in an infinite loop for the same chunk.
Fixes #1321, #1332

The loop would occur during loadChunk when Chunk.setChunkBlockTileEntity was
called. Since many chunkloaders override TileEntity.validate() to signal
ForgeChunkManager.forceChunk, it would hit our loadChunk method again and
get stuck in an infinite loop.