Don't enforce server paradigm on devices

[mlagally]

Use-cases which involve multiple devices, e.g. multiple servers don't require adopting a server model in the thing. Instead they take the active, i.e. client role and initiate the connection with the cloud. This improves the scalability and is a requirement for several deployments.

[mlagally]

Todo:

• need to add a requirement: endpoint devices can be clients or servers
• add language to servients chapter 8

[mlagally]

PR: #173

[mmccool]

PR #176 also addresses this (partly) in section 9. Some more figures would help in the final version; there are some scenarios I added (eg non-editorial context...) as "in more complex cases" paragraphs.