The class `EncryptedP2PState` stores the 4 32-byte keys, session id,
garbage terminators, whether it's an initiator/responder, whether the
initial handshake has been completed etc.. It also contains functions
to perform the v2 handshake and to encrypt/decrypt p2p v2 messages.
- In an inbound connection to TestNode, P2PConnection is the initiator
and `initiate_v2_handshake()`, `complete_handshake()`, `authenticate_handshake()`
are called on it. [ TestNode <----------------- P2PConnection ]
- In an outbound connection from TestNode, P2PConnection is the responder
and `respond_v2_handshake()`, `complete_handshake()`, `authenticate_handshake()`
are called on it. [ TestNode -----------------> P2PConnection ]
