Tracking issue about todo in distributed wal

[Rachelint]

Description There are some enhancements in wal needing to do in the futures, just mark them now.

Proposal

On message

• [x] Now manifest based on wal need to scan twice from storage while doing snapshot. Add a cache for this may be better.
• [ ] In wal's message queue implementation, we now sync the snapshot to remote in every flushing. Indeed, we should just need to do the sync work while flushing after dropping table.
• [x] Now, the desigin of wal on message queue is based on the thread model of one writer for one table, maybe we should remove this assumption.
• [x] Define some iterators with more explicit semantics.
• [ ] Now we init region(heavy work with network IO) even if it is just created, we should change MessageQueue to know whether it is a new region.
• [x] We should disable the ttl of topic in kafka, but it is not supported in rskafka now. It is important so I plan to use another crate to finish it temporarily.

On OBKV

• [ ] Rename the table name prefix.
• [x] Change the meta table's sharding key to table id.

Manifest(on wal)

• [x] Use the table id as region id.

Additional context