Enhance modularity of the specifications

[ptitjes]

Currently, the specifications of the messaging patterns define how these should behave but the main ZMTP specification defines the commands used by those messaging patterns. We could enhance the modularity of the specifications by moving the definitions of the commands used by each messaging patterns in their own specification.

The ZMTP specification would define:

• overall protocol (greeting, security handshake, and traffic = (command+))
• framing of commands and messages
• connection heartbeating

Each messaging pattern specification would define:

• the behavior of the peers
• the commands they use

This would allow to add new messaging patterns without having to modify the base ZMTP specification.

I am willing to make a PR to apply those changes.