SQLite I/O no longer runs on the main server thread and the shared
PreparedStatements are confined to one thread. savePixelBlock captures
a snapshot of the block data on the calling thread. Main.pixelBlocks is
now a CopyOnWriteArrayList and list mutations moved out of async chains.
Blocks that fail to initialize on startup are skipped with a proper
error log instead of being registered half-built.
MineTec