One party initiates the breeding; the other party exerts most of the
energy if its breeding key matches. It is that party's responsibility to
set up the newborn agent's connection (but the father can pass it an
arbitrary message). Newborn is spawned immediately but can be
renegotiated on first connect.
Agents do not die immediately on disconnect anymore. Instead, agent_id
is sent at birth and can be sent during negotiation to attach to a
disconnected agent.