Refactor the cluster metadata manager.

[chunshao90]

Description With the addition of the cluster scheduling module, the current cluster metadata management is somewhat confusing.

Proposal

• [x] Define the memory data structure corresponding to pb.
• [x] Refactor MetaStorage interface with the memory data structure instead of pb.
• [ ] Modify naming of struct in storage pb to make it easier to understand.
• [ ] Extract cluster metadata management into a separate module, simplifying cluster module operations.

Additional context