feat: NSQ cluster or public network message synchronization

[godcong]

I now have such a demand, and some nsqds are running on public network servers in different countries.
When I publish a message on a certain server, I need to synchronize it to all machines in different regions.
However, nsqlookupd cannot do this.

I want to upgrade nsqlookupd to support these functions.
Or create a new project.
I hope to get some suggestions from you.

[mreiferson]

dupe of #510