Modify the exist wal interface to adapt the new design

[Rachelint]

Description
Now the wal interface's design is for the mapping one table to one region (wal unit). For adapting to the new wal implementation based on kafka and solving some exist problem (for example, replay is too slow), we plan to modify the wal interface.

Proposal

• Make region id to WalLocation (include region id and table id now).
• Try best to keep current process in other modules (in fact, I prefer to add new components for avoiding direct interacting between Instance and WalManager to keep current process in later development).
• Add a new method scan to WalManager(implement it in wal on kafka firstly, other implementations will implement and make use of it after).

Additional context