two-process interactive shell progress * execute payloads (e.g. page/pinfo output) are displayed and paged. * aborted replies don't crash frontend. * intermediate iopub is displayed as it comes * wait for kernel to start before printing first prompt. This gets the right number on the first prompt, and also avoids the log output from being drawn after the first prompt. * protect most of the interact block from keyboard interrupts, which could cause weird errors if ctrl-C was held for a while. * separate restart/wait prompt when kernel dies.