feat: Store SourceStates in log and cache

[chubei]

This PR is a preparation for solving the log data loss problem.

• We store SourceStates in cache, so when cache reconnects to a log, it knows whether it needs to roll back.
• We keep track of the SourceStates that was persisted when a Log is restored from checkpoint, so a reconnecting client knows where to roll back to if necessary.

[Jesse-Bakker]

For my understanding: what's the reason for storing log offset and source states in separate lmdb databases?

[chubei]

I planned to store the source states as key value pairs in a database, so log position has to go to another database, otherwise there needs be some magic key for it. Later I found it too troublesome and went with simple bincode serialization but forgot to merge them into the same database.