refactor: chunk service & world generator

[smartcmd]

Reason

Currently, the chunk service implementation used in Allay is fragile and cannot meet our requirement:

• In a concurrent environment, the current implementation still has problems, although it runs normally most of the time. Several attempts show that we need to redesign the chunk loading process to solve the thread safety problem.
• Half-generated chunk won't be saved. The world generator is not stateless: it will save half-generated chunk for population usage.

Idea

• Make world generator stateless: chunk service should hold all loading and loaded chunks.
• WIP