`TestNode::add_outbound_p2p_connection()` is the only place where
addconnection test-only RPC is used. here, we always pass the
appropriate v2transport option to addconnection RPC.
currently the v2transport option for addconnection RPC is optional.
so simply make the v2transport option mandatory instead.
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 ]
