Message security issue for demo

[joyoFeng]

I found in the demo that messages are sent directly to the pool and then fetched. For the participating nodes in MPC protocol, their P2P messages or broadcast messages, if there is no message agent, need channel encryption, such as SSL for bidirectional channels; if there is a message agent then need message encryption, and then send through the agent (or again SSL)

In practical applications need to ensure the full process of cryptographic operations peer-to-peer communication security, external documentation should be detailed tips (from PlatON cryptography team)