Add Key Exchange Flow in BOLD #2

[kilrau]

1. Sita sends the first SessionInit message to Rama
2. Rama responds with SessionAck message
3. Rama sends SessionInit message to Sita
4. Sita sends SessionAck message to Rama
5. Sita calculates shared key_S based on Rama's SessionInit, Rama does the same. Rama calculates shared key_R based on Sita's session init message, Sita does the same. Sita uses shared key_S and Rama shared key_R for encyrpting messages from there on. The derived key for each "init/ack" interaction is used to encrypt traffic going to the initiator of the interaction. So the side which sent "init" is using it to decrypt incoming data, and the side which sent "ack" to encrypt outgoing data.

Details: https://github.com/bitcoin/bips/blob/master/bip-0151.mediawiki