Some new features and fixes:
- Make the process of install snapshot strictly sequential. Additionally, add BufferedSequentialReadFileAdaptor and BufferedSequentialWriteFileAdaptor for users to implement byte stream snapshot more easily.
- Store high-frenquency status of Replicator in a sperated structure, to reduce the lock contention with |_send_entries|.
- Fix some cases that leader lease rejects vote or transfer leader.
- Some other bugfix.