Refactor the cluster metadata manager.

[chunshao90]

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

Proposal

• Define the memory data structure corresponding to pb.
• 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