[Deliverable] Specify private chat protocol

[plopezlpz]

Summary

Understand and document the current Status private chats protocol stack. This includes encryption, usage of content topics, message flows, control message - size, frequency and purpose, MVDS usage.
This acts as the basis for future work, to modify the one-to-one chat protocol to stabilize, secure and scale it.

TODO

• Document the current protocol as it is used currently
• Improve and define specs based on the current protocol, keep in mind reliability, encryption, bandwidth usage and DoS protection

Tasks

• Document current encryption in Status one-to-one status-im/status-go#6319
• Document current usage of content topics (one-to-one) status-im/status-go#6320
• Document current message flows (one-to-one)
• Document current control messages: size, frequency and purpose (one-to-one)
• Document current MVDS usage (one-to-one)