* jupyter/kernels.scm (kernel-socket-kind): New procedure.
(zmq-poll*, allocate-connection): Export.
* jupyter/servers.scm (%server-scheduler-prompt, %server-exit-prompt):
New variables.
* jupyter/servers.scm (monitor-client, leave-server-loop)
(serve-kernels): New procedures.
* tests/servers.scm: New file.