Eliminate duplicate protocol info

[mcqueary]

The NATS Server clustering protocol duplicates most of the NATS protocol page. These can be consolidated to avoid duplication. If any protocol message has a special meaning when exchanged between servers (vs. between client and server), it should be noted in the main document.

[ColinSullivan1]

@mcqueary I did this with some intent. The protocol won't change much, so this doesn't pose any real maintenance issue, and if it does, there may be subtleties between the client and server messages. While they happen to be similar or identical in cases, combining the client and server protocol pages may be confusing. That being said, if you do find a way to make it clearer, absolutely feel free to reorganize.