* guix-jupyter-kernel.scm: Use SRFI-19.
(session-id): Adjust accordingly.
(<proxy>): New record type.
(register-container, container-by-name, container-socket-by-name):
Remove.
(register-proxy, proxy-by-name): New procedures.
(reply-execute-request): Adjust accordingly.
(kill-containers): Rewrite using 'vlist-for-each'.
(new-container-connect): Return a proxy record.