nsqd: raft-based HA message persistence

[andyxning]

I have no idea about how to do this for now. But, it seems that more and more popular distributed storage system is relying on raft.

Any plan to add this functionality?

/cc @ploxiln @mreiferson

[Telavian]

I am sure this is not easy however it unfortunately is a requirement for me.
I have a cluster of machines and HA storage with persistence is a requirement.

Otherwise if a machine were to crash then all the data on it could become lost.
Of course the drive could be replicated somehow however overall that is not a scalable solution for my case.

[mreiferson]

Dupe of the generic #510 and #625, closing.

Also hi @andyxning!