ipc transport: use Jupyter's {ip}-{port} socket address format #134

[paulk-asert]

Fixes #134.

For the ipc transport Jupyter treats the connection file's ip as a base path and names each channel socket {ip}-{port} (that is what jupyter_client writes and what frontends connect to). JupyterSocket.formatAddress used : for every transport, so under ipc the kernel bound …/kernel-abc:1 while the client waited on …/kernel-abc-1 and the frontend hung at "Connecting".

This has been invisible so far because the released JeroMQ (0.6.0) emulates ipc:// over loopback TCP and cannot talk to libzmq clients regardless. JeroMQ master (zeromq/jeromq#998, merged Sep 2024, not yet released) adds real Unix-socket ipc on Java 16+; with that on the classpath this address format was the only remaining failure. Verified end to end in Google Colab's public runtime image (which launches hosted kernels with --transport=ipc): with this change plus JeroMQ master, a jjava-based kernel replies to kernel_info and executes cells over ipc.

Change: - separator when transport is ipc; tcp unchanged. Unit test added for both cases.