There is no notion of unauthenticated connection, since the function
`authenticate` is immediately called on a
`P2p_io_scheduler.connection` and returns an authenticated connection,
or nothing. So, we only deal with authenticated connections. The
identifier of a connection is the same one as the one of the
`P2p_io_scheduler.connection` underneath.
Notes:
1. P2p_socket.Rejected -> P2p_errors.Rejected_socket_connection
to avoid conflict with P2p_pool.Rejected
2. Connection_closed error in P2p_pool unused and redundant with one
in P2p_io_scheduler
