Some useful features in NuRaft

[killme2008]

https://github.com/eBay/NuRaft is a raft implementation in cpp opensourced by ebay. It has some features that jraft doesn't have but may be useful, we should consider to add:

• [x] Priority-based semi-deterministic leader election . #264
• [x] Read-only member (learner) #115
• [ ] Object-based logical snapshot
• [ ] Custom/separate quorum size for commit & leader election #265
• [ ] Asynchronous replication
• [x] SSL/TLS support