Top supervisor for macula_peering.

Hosts the dynamic conn supervisor under which one macula_peering_conn gen_statem is spawned per peer connection, and, started before it, the owner of the station's session proof budget (macula_session_proof_rate). This process owns the table that says which handshake version each peer is dialled with (macula_peer_versions): it lives as long as every connection that reads it, so no child's exit forgets a node seen on v5 (macula#50).